To upgrade audio you basically need to replace the head unit, as well as the cruddy Subaru speakers (they’ve always sucked so, so bad). For a real kick, look into a sealed under-seat subwoofer unit to help compensate for Subaru’s perpetually terrible door cavity resonance that makes the door speakers have zero bass- but this part is optional depending on how much work and $ you want to throw at the issue.
Sure, you could probably replace all your speakers and add a dedicated amplifier for a hack solution, but you are still limited to the factory radio’s output quality (not great) and Subaru’s terrible nav system.

The good part is basically any modern touchscreen head unit running a mobile Android flavor will support Android Auto or Carplay so you can you use your far-superior smartphone navigation rather than the Subaru built in nav. Some higher end ones support their own dedicated nav, if you’re not a “connect the phone to everything” type.

The other good part is, based off some cursory Google, these older Subaru’s use a simple analog composite camera for the backup camera, and not a digital one run thru body communications- this means any aftermarket radio should simply connect to the new HU and work as expected.

Any kind of HU replacement is going to require some rewiring to go from the Subaru factory harness plug to an aftermarket radio plug. If you’re not comfortable using solder & heatshrink (or solder-and-seal connectors) that might be something an audio shop will have to do.
It will also take extra work to retain use of the steering wheel controls if you have them- I think there are universal converters out there for steering wheel signal wires but I have never had a car with any of that fancy stuff.

I’d recommend going through https://www.crutchfield.com/ and either using their self help guides or calling their service center with your wants and situation, and have them set you up with a good parts list. They’ve always been amazing to me and I highly recommend them for any car audio needs.

Personally, I am biased towards JVC head units, and Kicker speakers for decent budget sound quality.