I don’t know where this fixation for trying to make every approach into a 3deg one has come from. Yes, it’s a standard approach angle if there are no other considerations but there are plenty of approaches to large airports which are steeper for good reason. Guess what, you just brief and fly them!
The same for ISA+ effects. If the procedure is 3deg at ISA, then even ISA+30 will only make it just over 3.3deg - three whites and a red on a 3deg PAPI. Again, brief what you expect to see, why it’s like that and what you’re going to do about it in terms of approach stability and when you transition to the visual segment.
Until we switch to geometric altitude as a reference, we’re stuck with QNH/QFE/QNE and temperature effects. Unlike cold weather corrections, which being additive make you safer even if you get them wrong, rolling your own subtractive adjustments has much greater potential for dangerous error. And for what gain? So you can fly exactly 3degs? Why...?