I remember being in the same boat. I still fill my slots regularly but now always have keys of all types spare “just in case” and often I find something else to do when they are full.
Don’t be scared about joining a kingdom. If you are playing enough to keep your 5 chest slots filled you will make easily 400+ crests a week without a problem. Keep an eye on the ingame chat and there are often people inviting people to their kingdom (some without crest requirement) and I think everyone who says “I want to join a kingdom” has someone replying saying they have sent an invite.
I also leave the events and PVP to play when my chest slots are full. Although you probably already do that 
This will also help you decide which minions to level up so you are more likely to get bonus keys from them. A Guide to Minion Stats
EDIT: Don’t forget to do the quest tasks either. You also get key rewards from there too!