I had the exact same issue—-iMac 2017, just upgraded to Ventura (far as this system can go) …ended up costing me! New Premiere, new Toast, and now find that there seems to be no version of iMovie that runs on Ventura (unless we can track down an old one). BTW, I was ALL set to jump back to Final Cut Pro—but guess what it won't run on—Ventura! So stuck with Premiere for a while.
I run an analogue to digital transfer service—still need to connect with the old equipment! Used to use iMovie for one purpose—to import from Dig8, Video8, MiniDV, etc. Have a soft spot for iMovie—it is the first program I used for video editing, years ago… It took me maybe ten minutes to understand the basic process of video editing!
Not a great answer—but until I figure out something better, going to use a 14 year old, flawlessly running Mac Mini with iMovie—import the old video tapes into the Mac Mini's open firewire port then transfer to iMac and Premiere for editing. Anyone has a better solution, please post. There are hardware devices also that will allow this kind of transfer to a Mac.