I have a kinda radical, but maybe cowardly suggestion for guild raid that I think would nonetheless solve a few issues:
Just make guild raid rewards based solely on your individual contribution. If you do a certain amount of damage you get a certain reward, end of story. If your guild manages to down the boss it's just a cosmetic achievement for bragging rights.
There's the issue of access to bosses. Since you cannot individually down a boss, how to access other bosses? You can do a combination of damage and achievement based checks to do this. If you do a certain amount of damage and complete a set number of achievements like not bringing a mystic or something then you are considered to have 'cleared' the boss and can proceed to the other bosses.
It's cowardly because it sidesteps the issue of guild building, but I think it may be better to have a casual guild environment that is not so cutthroat as it is now. Everyone can just be friends together without resenting others for not putting in damage or kicking people out.
You can have players who clear all the achievements for a certain boss grant a debuff on the boss which will allow weaker players to clear it more easily, like the Tower event. You can retain the guild ladder for an additional bonus.to entice stronger players to stay in guilds with players who have less damage potential, Yeah this a little smelly because stronger players are kinda farming weaker players for ladder rewards, but if they provide a boss debuff, I think its fair.
I think it's better to build an ecosystem where stronger players can help out weaker players, rather than grouping all the strong players together with the weaker players no chance to advance.