iMac (Retina 5K, 27-inch, Late 2015) upgrade

Hi everyone!

I just found this and wanted to ask if someone had an experience with upgrading the iMac (Retina 5K, 27-inch, Late 2015). The model number is A1419.

What macOS have you tried to upgrade to?

Where can I find the mac to macOS compatibility table/chart?

I have a 2Tb Fusion drive, have you replaced it with SSD? Are there any video tutorials have you used?

TIA