I’ve had some time to work on this a little bit more. Basically I’ve found these threads:
Per the second of these threads, I had tried encrypted LVMs. Encryption sounds like a great idea to me. However, if it’s the problem… It’s the problem.
Turns out it IS the problem! I was able to get Yunohost installed with just the full disk - but could there be a better approach that would work for me? (Is it manual partitioning? I am willing to go there if necessary…)