Late 2015 27" iMac To Sequoia

I've been thinking about upgrading my Late 2015 27" 5k iMac (24GB, 4GB GPU, i7, 512 SSD) soon sadly though I'm not really wanting to spend the money right now, maybe in the new year after the new mac minis and studios are announced. I would jump on getting one now but the fact I have to get a monitor adds a lot to the $$ , especially after being spoiled by this 5K glass monitor.
It's been great though today when I tried to install the latest Adobe Camera Raw I was greeted with a message saying my OS is too old... yikes.

So I came across a couple great videos talking about OCLP to get Sequoia installed and many say it works really well.

I used to build PCs for many years up until about 9 years ago when I got my iMac. I was always installing OS and such, and always started from scratch when installing a new OS instead of just upgrading and keeping files.

I know ideally I should start from scratch but is upgrading and keeping files OK too ? I guess I don't have that much, just Photoshop and Davinci Resolve and some small utilities.

I kind of worry more about losing passwords and such but I guess those are stored in iCloud with Safari.

Btw, I remember once reading about how if you sell your mac and want to clean the OS, you need to log out of all your stuff like Apple ID first. Is that the same for something like this? Or it doesn't really matter since the hardware is the same (ie I'm not going to a new computer).

I don't use Time Machine, but I do backup my install files for apps and such on external hard drives.

Kind of nervous but I've heard good things.

Thanks a lot for any help.