Here's how I would have it if it was up to me
Bin the idea of declaring war
3 castles on fixed evenings, every guild is automatically enrolled in the war
Monday - Bichon Wall
Thursday - Phoenix Wall
Sunday - Sabuk Wall
That limits wars to 3 nights a week and lets people do whatever they want on other nights. They would all be points based, and every guild is welcome to try for them, just turn up and you start getting points, no need to declare in advance. So if the big guilds aren't interested in the 'less useful' castles of BW or PW then a smaller guild might get a shot at it
I selected those 3 because I personally find those ones the most engaging to fight in and require at least a bit of skill. The rest are just a measure of brute force
Meanwhile the other 4 castles would be awarded to arena participants
1st place gets Tristam
2nd place gets Arcane
3rd place gets GI
4th place gets WW
So the busy pve guilds have a good chance of having a castle each week
Feels like with a system like that everyone can have a chance to get a castle one way or another, and it'll be impossible for 1 guild to have more than 4 castles, which keeps them spread around