I am still running Ventura with latest updates on my mission critical Mac Studio M2 used in my music production studio. It appears all the software that I use is now Sonoma compatible. So. I'm ready to upgrade to Sonoma. I have the computer backed up with Time Machine on an external drive and have a copy of "Install Mac Ventura.app" on a SDXC card which was formatted and loaded by the computer. I will ensure that the Time Machine back up does not get tampered with by backing up Sonoma, once it's installed, to a different external disk.
Is there any other actions I should take to make returning to Ventura easy and simple if I need to?
If the only backup you have on your "mission critical Mac Studio M2", then whether or not you upgrade to Sonoma, you are flirting with disaster. Minimally, you should have at least one full backup (I like Carbon Copy Cloner, but there are plenty of others). Additionally, you should have another backup off-site (think natural disaster destroys your computer and backups) and one in the cloud.
Now getting to the question. To be able to migrate back to Ventura from Sonoma, the only way I know to do that is,
1) Before migrating to Sonoma, update to the most recent version of Ventura and make a bootable drive or USB of it.
2) After updating, run your backups (you should have at least one stand alone backup drive-preferably more and a Time Machine
3) Put those backups in a safe place, disconnected from your computer as you'll need them later.
4) After migrating to Sonoma, assuming you want to go back to Ventura, Do a backup of any files (music, documents, photos, etc. that have changed since Ventura.
5)Take your bootable installer and wipe your computer's hard drive.
6) reinstall Ventura.
7) load everything preferably from your Ventura backup or else use Time Machine, both via Migration Assistant. Bear in mind that any information you modified while using Sonoma will not be included in the restore from the Ventura, so you can manually move those files over from your Sonoma backup.
Essentially, you are starting as if a new computer and migrating the information. Just as with any project, the planning ahead of time (backups, downloads) will make it run smoother with no data loss.
Good luck