For Blutooth, get one of these. Have it in both of my 2004 Legacy wagons with Mcintosh stereos and it works fine for music via bluetooth. Hands-free calling is also passable.
https://www.aliexpress.com/item/Yatour-BTA-Bluetooth-car-kit-Hands-free-phone-call-A2DP-music-streaming-adapter-fits-Subaru-and/32566158650.html?spm=2114.13010208.99999999.261.MVxxqj
Otherwise you spend double that on an "Anycar" BT module.
You can replace the system entirely, but because the fascia is integrated with the AC controls, you will probably need to drop ~$500 on a new double din fascia before you even get a new head unit etc...