Absolutely love the work your doing Drunk Flamingo! I would put my money on CA copying this somewhat into a future patch!
Regarding the mechanic when reaching 100 loyalty with electors, i someone like it but i would've rather have them reaching 100 loyalty and than have the option to confederate with them manually rather than have it instantly on the turn.
I've not figured out how to vassal a elector yet but that would be preferred solution aswell when reaching 100. Not sure if you can confederate a faction if its vassal:d, so that maybe brings up some issue with it.

Keep up the good work, excited what you will bring next!

As for reaching 100 standing there could be pop out with 2 or 4 options.:
Confederate faction
Make them vassal
Something with military alliance but im not sure what could be put here.
Leave them as it is.

First two mechanics are probably best, you have option to confederate like it happens now automatically but also there is another way. What i consider the biggest plus of this mod is the fact it allows me to play as leader and defender of empire without the need to govern everything. I can run with my 2-3 armies and maintain empire as it is. With income from elector counts one province is enough to support all of this and reikland is big enough to build everything you need.
Vassal option would allow you to cordinate the attacks on your enemies.

Really should change the 100 loyalty thing to a choice, or default it to vassalage.