Mathesar core team member here. I don't want to distract from Outerbase's launch, but if you have time, would you be able to open an issue for the crash at https://github.com/centerofci/mathesar? We're still in alpha and relying on people checking out the project to report problems like these so we can fix them.

Also, relationships without foreign keys is on our roadmap: https://github.com/centerofci/mathesar/discussions/2276