iMac (Retina 5K, 27-inch, Late 2015) running 12.7.6 Monterey

I have a iMac (Retina 5K, 27-inch, Late 2015)

3.2 GHz Quad-Core Intel Core i5
16 GB 1867 MHz DDR3
AMD Radeon R9 M380 2 GB

I love the screen and it's still in such good condition.
It's still running 12.7.6 Monterey

I was going to just use it as a glorified DVD player because of how great the screen is until I discovered OCLP on Youtube. I'm seeing a lot of people putting the latest software on their old macs and I'm really tempted.

Because i'm nervous I wanted to ask a few of the people who know more than me.
What would the best operating system be for my specs?

I mean I'm pretty happy with 12.7.6 like it runs fine as a secondary computer but will I see any tangible benefit running Sequia or Sonoma? Will my old hardware be able to cope will it be better?

It's pretty fast with doing everything I want it to do now so if it's not worth upgrading I'll just leave it as is but please any advice would be awesome!