Hi Ben. Building an off back set of t206 is a super fun project and very doable (except, as mentioned above, the Monte Cross card). I put together a 520 set over the course of 4 years, and it was far and away the most fun I’ve had building a set of baseball cards. I think the harder advertising brands make the set so much more appealing…and they’re fun to hunt down. Even though my set is complete, I still try to upgrade cards to harder backs whenever I get the chance.
If you decide to take on the challenge, feel free to reach out…I’m happy to offer experience on which ones are tougher, etc.
Here is a link to my set
https://www.flickr.com/gp/146321797@N05/kh9s9T1X90