My question is how much do I need to focus on balancing the weight between the two rear panniers?
I'd say not much, I base that on thousands of miles commuting with one pannier, quite often heavier than either of my touring panniers.
OTOH - If it's no big deal and a matter of organising once and sticking to it, then balance might be a slight advantage, certainly no disadvantage. Balancing front panniers is supposed to be more important, even then many riders don't give it much consideration.
the toolkit goes at the bottom of one pannier.
I split my tools into two, the stuff I might need (Multi-tool, puncture kit, chain oil) and the stuff I probably won't. The first is easily accessible on the bike, the other at the bottom of the pannier.
I like to take a day bag, saddlebag in my case, but there's plenty of choice. In it I pack everything I might need while riding, plus a bit of capacity for shopping. If I end up staying somewhere more than one night and go for a ride, I don't have to think about what to pack, it's already in one bag. Plus, even when moving on, I don't need to open the panniers till I reach the days destination.
You'll probably find what works for you and it may not be the same as what works for me or anyone else, though I have toured with very experienced tourers who still spend an extraordinary amount of time faffing because they don't know where they've put something!