Hello !
Well, let insert my opinion about actual fort system, from the "other" side.
I understand and partially share OP analysis about actual fort system : It is highly perfectible.
Now, about some proposition like instances, level limitations, rewarding, our opinion gonna be very different. My post was supposed to be pretty short, and I realize by writing that he is well beyond the fortresses themselves. Sorry by advance for this wall of text... ...but i think it was important for me to say all this.
Instance is a false good idea. Why ?
Actually "issue" is population imbalance. When you have 20 vs 200, instanced or not, it will change nothing. Oh yes, some will be able to find an empty instance to insta win and redo, having those precious medals and gear in no time. No need to say that I got this feeling sometime, that some players would prefer to have a free shop, with insta max character max gear instead of really playing (remember BO ticks?).
Gear. I'm surprised no one pointed that. In the end it goes faster to complete a full invader set than a Vanquisher one or a Conqueror. Probably no one noticed. Mmm. I don't even speak about lowbies starting to collect this set even they are still in devastator. Point is, this part of the process has not been addressed yet.
Gear, leads me to contribution and rewards in general. The actual system is more or less the same you can experiment in a kindergarten, even you lose, you win !

. Starting a convos " Because i spent 1 hour to do this, i deserve atleast this". Well, this is true for a job, not for a game. I guess you never played Poker.

. Now, we know what would happen if loser were losing for real. A guildy told me that having only 500rp in lost a SC (500/30) was terrible because of the differnece between winners (7k) and losers (0.5k), because he "wasted" his time in it. I asked him what would be a correct reward for 15 min playing? "Oh like 2k minimum". Ok. Why not 10k? 100k? a full sov set? And in the end, why not waiting at WC in sc gaining stuff doing nothing ? Imo, it leads to wonder about expectation, behaviour and players mind again. Ofc, we will have to finetune contriution system, and there will be a moment it will be done, but don't expect because you lose, to gain as much as winners. Again, it is a faction game, and your reward depends on your faction motivation first. Which leads to population.
Population imbalance/X-Realming (supposed or not). Well it is a problem. There is a soft lock system, preventing a player switching faction to join the most populated. Point is it works for all characters on the same account. If a player have 2 separated accounts, ofc, it doesn't work. Is that our fault? i don't think so. How are we supposed to avoid a player having multiple accounts? My kids have one, got one. 1 home, 3 accounts. We don't know who is behind the screen. Now, to those who have on their own multiple accounts, question is : why ? divert the system and our attempts to balance what we obviously can't ? I'm interested about community opinion about it.
I'm not sure you would enjoy to be forced to play a side specifically BECAUSE the game needs it at one point. And in a OPEN realm vs realm game, starting to evoke any limitation smells like a CLOSED realm vs realm game. Is this what you want? I don't think so (Or maybe for others, not for you).
Imbalance. The same you can encounter in ORvR tbh. Years ago, community was crying to have multiple RvR zone opened in the same time, because of zerg, because of "strategy", because of "war is everywhere", because of " put whatever you want". And we listened and tried to do it. Result? everyone stay in the same area. Always. Sticking and sticking again. Fun fact,some have the nerve to accuse us to encourage the zerg. But some areas are still empty (Oh wait, let me guess. Lack of shinies...). So, if we had multiple forts in the time instanced or not, the same behaviour would be reproduced : stick and zerg, avoid fight as much as possible, looking for path of least resistance ( empty city siege syndrom), with one goal : Loot & Rewards first.
Speaking, the zerg, I take this opportunity to make an aside. I'm always astonished when some ambitious premades complain, that orvr is a total **** where they have no place. What do they expect exactly? an area with 2x12 ppl? making Praag a 6v6 place? There is place in this case; it is called scenario. Every evening with guildies we are in the lake, 4-7players looking for interesting fights. Like fishing, it is not every days a real success, but we find good fights, good opportunities vs biggest fishes, taking advantage of allies or not, depending situations, chasing AAO as much as possible.
You have to understand
we can't balance population without depriving you of your freedom of choice, and in my very honnest opinion you are not ready for this, we are not ready for this too. Simply because it is a game. Just a game. Call this bad design or player choice or maybe both. At a personnal level in a game likje a MMORPG, i spend months to make the character i like efficient inside a group, i don't wanna waste my time on an alt of the other side JUST BECAUSE the game needs it (even sometime i do; but it is my responsible choice...). Now question is, shall you blame those who are lookin for "easy" game? Maybe. Maybe not. They probably play a different game than yours or mine in fact.
You have to understand
we can't change player's mind. Some consider their play time like an investment, looking for profit and low loss. I just have an answer to those people, the same i have to my kids cheating to win, playing monopoly : In the end, thats just a game

. Losing a good fight is always better than winning a bad one. You'll remember the first for long, never the second one.
Of course, in this gigantic work in progress, there is a large place to enhancement and we are trying very hard to make it better week after week, at a pace you may consider as very slow (but who in fact is not at all). No need to say i'm shocked when some complain we are doing tests on live who are going wrong

. This is just RoR, this little work in progress project without pretention; not a AAA game :p.
In the end of the day, none in this place have the correct answer, player or dev, cause there is probably no perfect answer. There are some choices who fits some and choice who don't, there are mechanics you/we can deal with -as a player- or find solutions to adapt, and there are mechanics we can -as dev- think as being the most suitable for the largest majority in the limit of what we are able to do.
To conclude on this large "off topic" (i'm convinced everything is linked), from my position as a team member as much as a player,
without any animosity against anyone in the community, i have the feeling (and it is not related to OP or those who answered this thread), there is a very hypocritical behavior about the way the game is played and why some "fail" to find their own place in it. Everyone should question himself about his own expectations and personnal behaviour in a game involving hundreds of people in a joint action before blaming others for his own faults.
Communication ingame is near to 0, as if everyone was alone and depositor of his own faction.
Now to come back on Fort specifically !
To begin with, we are talking about forts. To be very clear if some missed the point, our goal is not to have 10 city siege a day tbh. It is supposed in a normal usage to be difficult to take.
Phase I : I agree. 15 minutes is too long. Even it seems to be too short -according recent epic fails- to be able to organize a faction and decide a strategy for phase II.
Phase II : I lost and won fortresses. Outnumbered or not. What i have learn : There are 3 doors, 2 posterns and 5 BOs. If you look carefully, each time it is lost, more than the number it is the lack of communication who ruin everything.
Actually, both faction are using brute force, even you wouldn't have to, according options you have. Fake push, splitting, flanking, sending scouts going on walls to announce placements or taking back flags, using postern to force defenders to move back, and take advantage of resurrection malus (it is the key, and it was designed on purpose). For this phase, space and map design is not really a problem, actually 30% of space is used, and options are numerous.
Phase III : We are aware this phase is not good in its state. Playing with postern is an option on the table, giving an active role to melee who can't do much waiting to enter as attacker, and same for defender who become more or less useless. Prison has flaws too and we are aware of. Some option not super precise atm are discussed. Removing it and place defenders near city walls ? Add multiple lever in different places? Change will come, like for everything.