Is upgrade to Ventura possible on an older iMac (Retina 4K, 21.5 inch, Late 2015) using external SSD?

Hoping the Apple Community Geniuses here can assist! Wondering if it's possible to upgrade an older iMac (Retina 4K, 21.5 inch, Late 2015 running Catalina v10.15.7) to Ventura using an external SSD as the startup disk?

Back in Aug 2022, "FirefighterDC" started a thread (here) that led to the discussion of upgrading his iMac (Retina 4K, 21.5 inch, Late 2015) to the latest OS. I have the same computer and wondered if it's possible to upgrade my iMac running Catalina v10.15.7 (Retina 4K, 21.5 inch, Late 2015; 8GB Memory; 2 TB fusion drive) to macOS Ventura using an external SSD as the startup disk?

In that thread, "Jack-19" outlined the steps necessary in his link: How to use an external SSD as your startup disk with your iMac or Mac mini. **Would these same steps be applicable?**

• • I have the equipment mentioned (e.g., LaCie rugged external drive (4TB) with USB/USB-C connecting cable; external SSD (1TB)) and already have iCloud+ for all my Apple devices so the iMac Documents & Desktop, etc are already stored in iCloud.

Final Question(s): The LaCie drive was used for Time Machine backups (TMB) on another computer, so wondered if it could be used with the iMac too?

• • Is it possible to use it as a dedicated TMB drive for the iMac and manually use it for TMB on another Apple Computer?
  • Is there a way to connect the drive to both computers simultaneously or should 2 different TMB drives be used versus switching it between computers?

Please state any additional equipment/steps needed if not mentioned in the article link.

Thanks in advance!