What's the "newest" OS that runs smoothly on a MBP 9,1?

Hey! Context: my laptop broke and my gf gave me her old mbp - it's a mid 2012 15" MacBook Pro 9,1 (Quad-core i7 @ 2,3 GHz, GT 650M). I have a desktop which does the heavy lifting, this unit serves me perfectly fine for browsing the web, coding and other minor office usage. The battery still holds up fairly well at around 4h of autonomy, more than I need.

I retrofitted it with a 240GB MSI S270 SSD and 8GB 1600MHz CL9 G.Skill Ripjaws, deep cleaned it and repasted the CPU + GPU with a decent compound. It runs perfectly fine under Catalina, way better than what I expected when it still had the factory HDD and RAM.

Thing is, I need it connected to the internet and given how old Catalina is, it's a major security concern. It's also very cumbersome to install apps, the app store does not work and most third party apps don't have an up to date version that runs on 10.15.7.

What's the most recent version of macOS that will run smoothly-ish on this rig as it stands?

TIA