Shubla has a point. As it is originaly proposed, it would make powerfullplayer even more powerfull. A possible midle ground could be:
Realm bonuses should be proportional to the player's contribution to it not a plain bonus proportional to it own stats. Devs could add a monument stone, or sacrifice stone player would donate stuff to gain more bonuses (capped for max realm bonus). The aforementioned "donation station" could be a personal one like fealty stones or even like the realm foundations stone only accessible in realm's capital. The last opion would incentive playr to build roads and travel more outside.
This way, if you want to have acess to realm bonuses you need to work for it. (similar to credo or quests)
The player would be invested in one realm, since it would be too much time to invest in more than one. Therefore the player would care about realm wars and such if he is interested in keeping his bonuses at the level they are.
It would, possible, make realm leaders care for their subjects more.
Ty for open your idea, don't get sad or too much annoyed by the ppl that don't know how to brainstorm and just go for: "no, cos no!"