Hi. I just upgraded to 2019 Ultra pack - lots of download time, etc... Now looking at doing a clean install of MacOS Catalina, and was wondering if I can just copy the entire BIAB folder to an external drive, and then back to the system drive after the OS install, OR if I need to start from scratch.

I've had my MBP for 7 years, runs great, but there's an accumulation of junk from simply upgrading over the years, so I think it's time for clean install.

Yes, you can do that. After you've finished reinstalling the system, you will just need to run the main program installer (just the one single Band-in-a-Box 2019 package), so that the fonts, plugins, etc. are installed properly. One tip, you might consider cloning your entire hard drive rather than backing up individual folders. You can do that using the mac's restore feature, or even better, use one of the third party programs like Carbon Copy Cloner.

If running from the BIAB hard drive, you can include that in a Time Machine backup through the Options menu in Time Machine. This lets you include or exclude folders or drives as you like. If you use 10TB drives in your backups as I do, the Ultra Pack takes up little space relatively.

OTOH, since everything in the Ultra Pack is downloadable from your user account, no real reason to include it in Time Machine or other backup.

The only reasons to move the Real Tracks and Drums folders (as I do) to an external is if moving them to an SSD. There's a slight but noticeable speed bump. In my case, I don't like my desk being cluttered with external HDDs when I can move Komplete, BIAB Audiophile (RT and Drums only) and most of my other VIs onto a single 4TB SSD.

I run the installer so that everything else is sitting on my System drive.