On the topic of broken parts, the 3D printed seat support that I'd made has also failed.
I think this is more likely easy to explain. The seat support gets repeated loading (and shock loading over bumps and pot holes) and it seems like this was an easy part to crack.
Despite it being easier to identify the fix, I think the easier answer here is to get a seat post that fits the bike and isn't bent and dented.
Figuring that out will be a bit of a trick because the inside of the seat post varies noticeably in diameter
I've already mistakenly gotten the wrong size once...
... Maybe I have the right size this time