I have a rb20det swapped into my '89 s13 hatch and one day when I parked the car to get some food I came out, started it and the idle sounded to be around 2000rpm (no tach :C). If I'm in 1st gear the car will drive itself at a constant 15mph without my foot on the gas.
So far I've cleaned the IACV, checked and set the TPS to 0.45v, screwed the idle screw all the way in and checked all over for vacuum leaks. My boost gauge reads 20 vac at idle most of the time but sometimes will dip to 15 and sound like a truck on cold starts.

Another weird thing is if I do a pull and get into boost when I slow down at stop light for example the idle will sound perfectly fine (under 1000rpm) until I shut the car off and start it again which makes me think it could have something to do with my MAF.

I've been searching all the forums I can about this but seem to only find decade old threads with instructions that haven't fixed my issue.
Any thoughts are appreciated and if there's any more info I could provide that would be helpful just let me know!

I was originally thinking vacuum leak somewhere. Is your timing correct too? What ECU are you running?

Running the stock r32 gts-t ecu, and I'm hoping it's just a vaccum leak hahaha

It's almost always a vacuum or unmetered air leak. The engine cannot run higher than normal other than by getting more air than normal. It has to have enough fuel in the idle map software too or it dies instead when the fuel runs out. Any idle setter like an IAC there can stick to do it too. Many miss the leak over and over until they finally find it. That car old enough that if it has any decel equipment (valve or solenoid) that can do it too.