Related RPS article: https://www.rockpapershotgun.com/2018/02/27/what-works-and-w...
I never was close to failing to finish a puzzle in OM. It was just a matter of how poor my solution performed whatever metric I chose to optimize first. The puzzles at the end of the campaign had me "give up" and settle for an ugly slow solution though.
> I feel no compulsion to return to OM after finishing the main campaign
If you haven't tried the Journal puzzles in OM, I would recommend you give them a shot. Many of them are in confined spaces rather than an infinite plane. I have yet to find a puzzle as hard as the ones in the TIS-NET directory, but they are a step up from the rest of the OM puzzles in terms of difficulty.
(Or even without friends, like me, I just refused to proceed to the next level until I was in the global top 1% for the current level :P)
The 3 saved puzzles is a bit annoying, but you can always backup the puzzle files since they're just text.