I think the best thing to do is use approximation. Just know where the rough range should be and decide if you need to bluff a bit more or take it back a notch.
So lets say you are on the button and just 3-bet the CO player who now makes a 4-bet. You can now 5-bet all-in with JJ+ AQs+ and AKo for value. For bluffs throw in a couple of low suited aces. Your best bet for 5-bet bluffs is to choose hands that block 5-bet calling hands and also perform well against them. 5-bet calling hands from that spot are usually JJ+ and AK.
As a nice rule of thumb. Try using a 1:2 value:bluff ratio for 3-bets. 6:4 value:bluff ratio for 4-bets. 4:1 value:bluff ratio for 5-bets. This is not exact, but it will be better for your game overall. Having a rough guideline will allow you to play in better accordance to reads of other players rather than confined robotic play.
I think going the game theory route can actually make you suck more at poker. This is just speaking from my experience. I know that I can get caught up in trying to play theoretically correct that I miss out on opportunity. For example, sometimes you see someone who 4-bets very seldom and accidently end up 5-betting all-in with A4s. Another spot where I may have A
9
on a triple diamond board. I'll overbet all-in on this river because I have the cousin diamond suit (hearts) with an odd ranked card which is to mimic what I would do with the nut flush at a 4:1 value:bluff ratio. This might make my value:bluff ratio a more balancy look. This overthinking can sometimes be a disservice, because you may make a bluff you strongly feel you will get called with and end up screwing yourself out of more money.
If you are a loose aggressive player you can sometimes get away with 3-betting a bit tighter and still have the perception of having a wide range.
For simplification purposes in your learning purposes, try not flatting any 4-bets. Either 5-bet or fold. This goes for IP and OOP. I am aware there will be many that will disagree with this advice, but I think overcomplication is a ticket to making you make mistakes.
There are some players out there trying to develop a donking range in HU pots. IMO, why bother. Keep it simple if you can. Don't make ranges for the sake of making ranges. Do what makes money and don't trap yourself in a poker box.