Just to update, I've got a smart phone (Samsung A20) coming, along with a Nulaxy KM18 FM transmitter.
This will allow Navigation on the phone, the FM transmitter plugs into the car's power outlet and takes the bluetooth signal from the phone and 'casts it to the radio for playback.
It also can be swapped into any other 12V car with FM radio so it will work in all the other cars too. No need to wire each radio for input.

The phone also accepts micro SD cards up to 512GB, so I can load up a LOT of music to take along. The phone can sort thru the files so you can select what to play.
The FM deal also accepts SD cards and thumb drives.

Reviews seem to be good on both and the FM deal is under $30. Yeah, it's cheap china electronics, but there aren't many other options to get this done.

Overall, this seems to fit what I want at a reasonable price and be able to do what I want in multiple vehicles.

Just figured I'd update you guys on what I've found. It may not work for you, but at least it's an option for older stuff.
Now I just need to figure out where the phone fits best while driving and get a mount.