Catalina to Big Sur?

I had no issues and haven't seen many (or maybe any) reported here. But a backup is recommended in How to upgrade to macOS Big Sur - Apple Support but I would always recommend that even if not upgrading.

Yes. For instructions, see: How to upgrade to macOS Big Sur - Apple Support

Pebblet wrote:

No I think you warned me before to steer clear! The only clean-up is Duplicate File Finder Pro.

Be careful with Duplicate File Finder Pro. I wasn't able to test it as it didn't have a demo version to try but you don't want one that does the deletion itself (in a Photos library or on the hard drive). You want one that just identifies the duplicates and leaves the deletion to you.

Pebblet wrote:

It should be smooth and no hiccups? Once bitten.....

As long as you've not installed and run any "cleaning", "optimizing", "speed-up" or anti-virus apps on your Mac?