So I was recently on DTNS talking about migrating away from TinyLetter.
One thing I forgot to mention, I found a way to export the content of newsletters! TinyLetter lets you easily export your subscriber lists, but otherwise you can only export the record of each newsletter being sent (time, date, title), but not content. Turns out you’re a wget command away. Tried it and it worked beautifully to back up over five years of newsletters.
TL;DR on the migration, ended up going with WordPress.