Quoted $2500 on a P0420, looking for el cheapo

Car is a 2018 Subaru Impreza base 2.0 with 91k miles.

Short story short, CEL lit up Tuesday, shop confirmed clogged cat and an oil pan leak today. Got quoted $2500 to fix, and I’m not confident the total won’t exceed that. I also have had very mediocre experiences getting routine car maintenance like brakes done and have little reason to believe my experience will improve with this go round.

My understanding is that given Subarus propensity to burn oil the higher mileage they get, this isn’t going to be the first cat that gets replaced on the car so I’m also questioning the wisdom of spending so much on an emissions part that will likely fail faster the second time.

I’d like to buy a cheap cat online for a few hundred and install it myself by cutting the bad one out and hose clamping the new one in with muffler tape for the gaps. It seems relatively straightforward and SIGNIFICANTLY cheaper both financially and emotionally.

Is there something I’m missing here? Something that would complicate this, make it inadvisable or not a working solution?