There's another option that I plan on doing once I get my vehicle as I'm partially invested for my G20 already.
AMP: AudioTec Fischer MatchUp 10DSP with the analog MEC - there's a forum user on the G20 forums that has a DSP profile as it doesn't come standard with it, but I think the company is working on a default one. It also allows for RCA out if you wanted to add an additional amp for some trunk subwoofers
I also plan on trying to make some fiberglass stealth boxes that fit in the spaces behind the wheel wells, that's a problem for future me to figure out once I have the vehicle or order the trims to mess around with while I wait not sure yet.
Speakers: Eton BMW speaker replacements that are pretty high quality and also plug and play they have option to replace all 10 speakers. While I haven't held the Hardon Karmon speakers in my hand the magnets are roughly 4x the size of the Top HiFi speakers in the G20 and the cones are considerably more durable. The underwear's pack quite a bit more punch than the stock as well. If I didn't primarily listen to and prefer bass-y music I'd say they what should have came stock as it vastly improves the soundstage and completely that flat tin sound. There testimonials on the G20 forum that when paired with the DSP amp above it's fantastic with the right profile for the car. They have a chart on their website that lists compatible speakers, but I don't know if they have a distributor in the US as I'm overseas for work