I just installed an OUT OF SIGHT AUDIO unit in my 911sc. Very discreet. Very easy wiring. Just a keyed power a ground and the wires to your speakers. I put mine behind the stereo block off plate. May be worth a look

What radio is on the car actually installed? Most Becker models are upgradable with Bluetooth. Mine (1972 Becker Mexico) is upgradable, so if yours is a Becker probably is possible

Amazon music on my phone played through bluetooth of the car

I'm considering mounting a small Wetsounds amplified sound bar under the dash so that I can leave the original blaupunkt cassette in the dash and not mess with the door/package tray speakers. Not cheap but I have heard great things about their products.

https://www.wetsounds.com/STEALTH-6-SURGE-B

Another option is to upgrade door speakers (can use drop-in, but higher quality speakers and not affect look/originality), run new wires from door speakers, and install an amp and bluetooth module under passenger seat (or in trunk, etc.) -- feed it with an iPhone/etc. and mount the phone using the rennline magnetic mount that attaches to the clock bezel

I went with the new Porsche Classic Radio - a bit pricey but I wanted to get rid of the old aftermarket Kenwood unit that was dying and go back to the classic Porsche look. Love the unit - use the Bluetooth mostly - upgraded my speakers to Polk Audio's - all I need now is a good low profile 4 channel amp. Any recommendations?

in my last M3, I removed the whole stereo (like my still-in-progress SC). I just used a JBL Flip 3. I bought a faux-leather cover that had a 1" wide strap on it and ran the strap over the bar nearest my head on my half-cage. Worked well for ~5-6 hours at high volume, longer if I played it softer. (which I didn't in the car because warmed over LS and no sound-deadening)
my plan for the SC is basically the same, sans the half-cage. I'll toss in my Flip 3 and sync it to my phone. If I find it's not loud enough, I can buy another JBL BT speaker and sync them to each other and then my phone for a multi-speaker system.
cheap, modular, easy to move/place, and if you have to run power to it for a longer drive, the cord is USB. I won't need it for grocery runs, so it doesn't need to be over-thought. Hell, it's just as likely that I'll toss it in the passenger seat unless my wife is with me.

I have a Braven 850 Speaker in my 20 x 30 shop and continue to be impressed by it.