Alright, so most people voted for Mojave. After thinking about it, I decided to go with Catalina as the only thing keeping Mojave is 32bit, and I don't think I have 32bit apps left. So I went with Catalina. Plus I then have an extra year of support left.
And trying to clean install to Catalina is not straight forward. 😀 Typical Apple.
Apple has instructions how to download and create a bootable flash drive for Catalina, but they failed to mention that High Sierra cannot download the full installation of Catalina. Only the stub. So I ended up "upgrading" to Catalina first, and then download the full OS, and re-format and reinstall Catalina from scratch, just to ensure a clean install.
So far it's going well, ie. I don't notice much difference with High Sierra in terms of performance. I appreciate dark mode, and the files-on-demand feature that is now available for OneDrive.
Thanks for the input guys. 🙂