I would install some better 5.25" speakers in the front with additional tweeters on the dash.
IMO rear speakers sound bad. The stage is in the front.
You can build a small woodn subwoofer housing inside the center console.
There fits a 8" subwoofer.
Subwoofer enclosure inside the stock center console?
With a 4ch amp with about 4x 75W you should be fine.
Thats 2x 75W for the front and about 1x 200W (bridged) for the subwoofer.
If you buy used parts all that should cost less than 100$.