Depending on how handy you are/confident you feel it might not be too difficult to remove the radio and check the insides, check if all parts look fine and move properly, apply a touch of grease when necessary.

Yeah lots of modern radios are touch. You can always go single din and just have the other slot empty/as storage, you can just buy an insert, they are cheap, and single din are usually more old school with buttons. Like this thing:

https://www.kenwood.eu/car/receivers/bt-receivers/KDC-BT460U/

I suppose the standard “fix” nowadays is not to use CDs.

As for actually fixing it, a slipping belt seems like the obvious issue, but no way to tell without opening up the player.

The radio is just a regular double DIN unit and easily replaced. You still want something for your car because you’ll get ugly gaps without the right faceplate, or you can just buy a bezel and slap anything in there.

Low oil doesn’t mean the car doesn’t work, it means the engine isn’t lubricated enough. This usually doesn’t cause immediate failure, but excessive wear. Your engine won’t fail now but it will fail much earlier if you don’t service it.