Unfortunately, I still have Windows XP clients, and I couldn't figure out how to join them to the Windows Server 2019 domain. The error was a simple "An internal error occurred." with nothing in the event logs. I thought the error could be related to SMB1, but it wasn't. I also thought it could be because it's simply not supported, but from Google searches it appears to be possible. It also didn't seem to be related to the domain functional level.
After months and months of hair pulling, I figured it out thanks to a seemingly unrelated blog post. The post talks about problems joining 2008 R2 domains. I had no issues with 2008 R2 since before the upgrade that was what we were using. But installing KB969442 instantly allowed me to join the XP systems to the 2019 domain (functional level is 2016 of course). Funnily enough the post was from nine years ago to the day. The blog looks abandoned like mine but I left a message there thanking the owner.
Now I can continue to use unsupported OS's.