So... 90mm intake is done, berklee is done, car is actually repaired and running correctly now... 19x8.5 wheels and expensive tyres done...

aside from supercharging... what's next in terms of squeezing a little more power out of the 380?

tiny budget initially.

Tiny budget? Power? What? :io:

As Neo says, other than what you have already done there is no more cheap power to be got from a 380.
All I can suggest is run it on 98 RON, such as Shell V-Power or BP Ultimate.
In theory the knock sensor will advance the timing a couple of degrees and give you a little more go.
Short of forced induction, you could try a piggyback ECU/chip tuned to run on 98 with advanced timing, like I did with a ChipTorque Xede.
But you won't get any change out of $1500 to get one properly fitted and tuned.
One thing I would not recommend is fitting RPW extractors, too little gain for the expense and hassle involved.

Yeah, those mods you did are the best cheap ones. Big dollars after that. You can always try that lower airbox resonator chop mod; a few people did it back in the day (I drove another members manual 380 with that mod and it does make a difference, mainly to sound). I didn't do it; too fiddly getting down there removing all those plastic screws... but for a temporary mod, take the intake funnel off the air box for a day or two. Car sounds awesome! I did it once* and was turning heads everywhere I gave it a bootful. *but on third day I did geta CEL on engine start so I chickened out and put it back on again.

I did removed the resonator box completely and it's been 3 months or so now...haven't had any issues or CEL on (so far!!)

It does make the gas paddle respond a bit quicker and more noise when reving over 2.5k (which is nice not too noisy) on a SX with only a K&N OEM air filter, so I think it might be a bit louder with the berklee