Playing vs the nit/tag

You are right that your raise is quite bad OTF. You usually don't want to raise on dryish flops like this because your range won't have many draws, and second pair is just bad to raise. And then there are the reasons you already said. Same applies OTT. Villain cbets multiway and calls a raise on that flop, so he probably is not going to fold much OTT.

I have noticed that many nits who play very few hands like to play their hits until showdown. I mean that when the guy finally gets AK and can play his hand, when he hits TPTK he isn't folding it easily. So I don't know if you can exploit these guys by barreling. Maybe it would be better to exploit them pre flop and then postflop do one and done. If they continue playing postflop their range is probably strong.

What about this for example, A nit opens we call IP on the btn he check calls flop, do you you think most nits ranges are polarised, to weak to medium hands, i think this could be a multi barrel oppertunity, what do you think?

Maybe, there are a lot of things we have to take into consideration.
If the board is drawy and we have top pair or better, I think it is ok to double barrel. If the board is dry and we have a weaker hand, I think it might not be as good. Nits can sometimes check call the flop with weak top pair too, and then it just depends on villain (and how he thinks you are playing) if he is going to call a second barrel also or not.

I guess my main point is that I wouldn't barrel everything. If you have a top pair -> you block his top pairs -> barreling might be better. Also if villain can have some draws on that board, even better. Or if you have a good draw yourself.