I answered in your other post, but I’ll add here as well.
You can rent an X30 or Rotax for a weekend as well. You don’t need to drop 10k on brand new everything right off the bat.
The thing about those multi-make series you mentioned is that the cars and engines are all designed to a set spec, so they’re all relatively similar and can compete together with minor adjustments to ballast or whatever. The mixed engine TaG class had a bunch of engines never designed to compete together, so it was next to impossible to balance. They tried for years and never really nailed it.
Also not sure where you get the idea that F1 is a “spec series”. It’s literally the opposite, nothing is spec minus a few auxiliary parts.