I was hoping to reinstall and use the purchased previous version of OM on my M1 Ventura Macbook, but under downloads I see only versions marked "Big Sur", "Monterey" etc. and not "Ventura." Is this possible?

You need Opusmodus V3 on a Mac M1.

Given that Rosetta exists, I don't understand why the design choice was made to force people to upgrade on an M1, unless of course it's free to go from V2 to V3.

 

Do a forum-search for m1 or Mac or "Opusmodus 3" and you will find all the discussions and answers from the past year(s).

Then you can understand the design choice and why the upgrade to V3 is not free.