I've had to make repairs to my phone a couple times, screen cracked once from a drop on tile. Then during that repair (screen for~$80, think I got lucky), the fingerprint sensor ribbon connector on the motherboard of the phone broke, after about six months I bought a parts only phone with a cracked screen for ~$110 and transferred good motherboard to frame with good screen, boom fully functional Pixel 3 XL.
As a cautionary message, the ribbon cable clamps, that the fingerprint sensor uses, are very fragile unlike the lego style connectors that every other connection in the phone uses.
My total repair cost would have less than $100 if not for that fragile connector.
As to the back glass mod, I had almost a whole phone's worth of spare parts so I started scratching the paint away and around the NFC coil, as that is adhered to the glass and cleaned the adhesive off the parts that are exposed, plenty of double sided adhesive tape later its all together and clear like Tom Cruise.
Both phones work too, so now I'm just looking for ideas for more mods, any ideas?