$30 6max, Q8s 3b Pot

just fold or 4bet this hand. QJs would be the bottom of my flatting range, but even that i might just 4bet if I think he is light.

as played, you obv just want to flat the flop nothing else makes sense.
yes you have to fold to a 2 barrel, but that isn't bad because he shouldn't be light 2 barreling this board because your flat looks like Kx so folding the absolute bottom of your range vs a super strong line from a competent player is completely fine.

What if we do have a TP in these spots, do I just call him down? (despite his strong line)

Yeah I think you have to call down top pair. The worst top pair you should have is KT. I can understand folding a KT maybe 1/10 times to this line if you get a feeling he is really strong (timing etc..) but if you are thinking about folding Kx often then your decision to fold that type of hand should be pre-flop IMO.

If you are having these problems post-flop vs a particular opponent you should be 4betting > flatting with this stack. You can make some serious mistakes with your range vs his range if you flat and get into a strange leveling war on dry boards. But you can never make huge mistakes pre-flop if you have a good idea of his value vs bluff 3bet range. This is particularly true against aggro regs who merge well because your positional advantage is somewhat controlled by their high bet flop/bet turn freq (you don't have the opportunity to hand read IP because their lines are so well balanced/merged).