As long as you're using Postgres, I would definitely try out an approach based on template databases in your situation. It should check all the boxes that a run with `db/seed.rb` would, except an order of magnitude or two compared to what Rails would be doing (i.e. parsing a big Ruby file and loading schema in table by table).

One of the best things about LLMs is they make these sorts of refactors entirely plausible even if you're not a subject area expert. You could probably prototype this and have a patch ready in an hour or two.