overall, playing in a balanced fashion is an important skill for a serious poker player to utilize. it's how we avoid being exploited by other good players. it's a skill that is very important in some games and in others, not so much. in a game where the situation you describe comes up often, i'm not sure balance should be your main concern. since it is something you're thinking about, think about this: if you do it right, your range will sort of balance itself out.
imo, post flop, in order to be balanced, your entire range doesn't ever really want to do anything 100% of the time. doing something with your entire range 100% of the time is an oversimplification of balanced play.
think about your range and then split it up into different action regions. a lame example i'll pull from my butt is as follows:
the range i raise after two limpers looks like this:
66+, A5s+, ATo+, K8s+, KJo+, Q9s+, J9s+, T9s, 98s, 87s
in the situation you describe vs. two limpers and two blinds facing a donk bet from the player directly to your right, i'd do something like this:
raise for value:
88, 99, AA, AJo+, A8s, A9s, AJs+, J
T
, Q
T
, 89s.
call (with the intention of calling down when the board doesn't get too nasty and raising if we improve. draws we'll fold the river unimproved):
AT, A5s-A7s, KK-TT, K
8
+, Q
J
, 8
7
, Q9s+, J9s+, T9s, 87s. (these 9x and 8x hands can be folded on the river ui depending on the board and action).
fold right away:
66, 77, KTs-KQs(no flush draw), KJo+.
raise as a bluff:
QTs.
now we naturally have hands that fall into every region and we're relatively balanced. this is just a very rough estimate of what to do with these hands based on nothing, but i'm trying to show that you don't need to think too hard about how to be balanced in this spot.
in a live 20/40 game, my guess is that you'll rarely run in to a spot where, even hu, you'll need to worry about being exploited by the other player. even where you play, where there are a lot of really good poker players in the player pool, you're just not going to be in a spot often enough where balance should be your main concern. i'm not suggesting that you never think about balance. on the contrary, i think playing as balanced as possible is a skill needed for a good player to make the leap to the higher games. it's just that getting donked into on this flop is just not something that's going to happen often when hu or 3way vs. a good player(s), when balance is more important.