This is really villain dependent. If their 3b range is AQ+ / TT+ then we have 47% assuming they are not 3b folding AQ or TT. If you throw in some ATs AJo and some light 3b our equity increases. IMO JJ is pretty much "the line."

Given we have position, I would be more inclined to take a flop and play postflop with position. Still with JJ overcards will fall often and we might get bluffed or might let a hand in that was folding to the 4b out flop us in a way we cant really see. Out of position adds to the merit of a 4b pre.

JJ: 52.3% vs AK, AQ, TT, QQ, KK, AA, JJ, AxTx, AJ, Tx9x .... Tx9x is to account for some bluff 3bs, likely we can add more as most competent players will be 3b more SB vs Btn.

I would not be jamming 55/66 or 4b jamming a5s/a4s. If I wanted to have some 4b bluff hands I would be 4b folding not 4b jamming, though here I do not think that is an option given stack depth and the size of the 3b pre. I would flat these hands and play position.

Is $4.50 your normal open here? I don't mind it but I think it is bigger than you normally would go pre?

Has JJ become too loose of a jam at 100nl BTN vs SB?

To me without any further reads it looks okay. As well as picking up a lot of dead money I do expect them to turn up with TT-77 and maybe even lower if there's a dynamic.

Looks like it was a limped pot with a re-iso, so MG iso-raise to 4.5x, which I like given the holding.

I agree with what Josh is saying. I think for JJ to be a profitable jam overall, you need villain to be calling sometimes with AQ and TT, or to be 3betting fairly wide. I have it as a stack off in my ranges but IDK, given that it was a re-iso. Not sure at your stakes and on N8 if they are re-isoing the same range SBvBU as a normal 3b range. If they see you doing a lot of iso-raising then maybe the are re-isoing wider, in which case jamming is probably good.

Pretty standard to get it in here, does look strong though, given our large iso though.
I'm almost always getting it in here calling a 4bet with JJ OOP with this SPR seems horrible to me.

With a read we could make the exploitive fold if he never has a bluff here. Like do we only ISO this size with a monster and is he a reg who realsies that? If so we could potentially let this go. Would seem really bad for his to show up with worse than JJ given that dynamic for instance.

Not an expert on this spot by any means though, it's not one we see too frequently.