Alright..
So instead of looking at the symptoms shall we look at the course here?
T2-T3-T4 middlezones give no incentive to players if they got vanq+ gear and their desired genesis + subj weapons apart from renown ticks.
On top of that the standard zonelocking mechanics are really predictable and boring with running resource boxes untill 2 stars, leave all BOs and everyone stay mounted behind 4 tanks doing a ramming minigame only to dismount every 3minuts when the oil is won by a single defender who clicked it first. Engaging gameplay? not really.
Allow the players to use abilities on the gates if the besieging realm hold more than 2/4 of the Battleopjectives, this will spread out the action in the zone and not just leaving us with the predictable "2star all neutral BO" siege situation. Letting players attack the door with very reduced damage will allow useage of a supperior number siege to rush down a zone and estabilish a foothold before the other realm can react if you do a coordinated zoneswap rush. On top of that, add a restriction so if the outerdoor is below 50% defenders can no longer respawn inside the keep, making the attackers able to spread the forces to cover the posern doors preventing reinforcements and defenders from entering the besieged keep, but again also potentially spreading the numbers and action out instead of the 100+ glorified ram-bodyguard team at frontdoor we see atm.
Alright, now we have more interactive and engaging zoneflips how about the rewards? Well get rid of the Sove requirement of Needing Invader ToK to wear Sove, and increase it to RR 80. And directly steal the chain-lock system Age of Reckoning had where the more zones you locked in a pairing the bigger bonus renown tick you accumulated when you finished pushing a pairing. Now players have intentive to stick to the realm and play the lower tier zones and midtier T4, the invader need is no longer as massive and the fighting is more engaging and based on coordination realmwide and creativity instead of a the dull outer-funnel-inner-funnel-lord-lock system we have atm.
Will some players jump on the bandwagon and join a momentum push, sure. As long as the server is free to play and allow crossrealming to some extend, there will always be this. Not only can players just make an other free account for the secondary realm, or use a VPN (it makes you bypass the realm lockout btw)

But the incentive for the "realmpride" is not there atm. And I suggest giving some sort of realmwide buff based on city star that is not too combat-related but also gives you a reason to stay and fight against the snowball push when zones are chainlocking and the siege is closing in.
3Star city: Fortress lord learns a new ability to make him harder and not a garenteed win as soon as attackers secure lordroom in a fort (aoe healdebuff or whatever you come up with)
4Star city: Scenario quest-turn ins in the capital now give 2.5k renown per turn in. or double medals from the PUG only scenario.
5Star city: Land of the dead is now open for your realm