Keep Monterey

While you could co-install Monterey and Ventura on the same disk, or put one on an external drive, managing them both becomes complex and error-prone.

If you still have apps requiring Monterey, run them in Ventura’s lightweight virtualisation with a free app like VirtualBuddy.

Their major limitations are that their virtual machines can’t run App Store apps, access iCloud directly, or do anything relying on your Apple ID. Most run at native speed, though, with full-speed Metal graphics and GPU acceleration.