Thank you for the comprehensive answer, exactly what I was looking for!Hi, and welcome to XDA!
Good question!
While it is "threatened" that Amazon refurbs will be updated, in practice, it usually is not the case. That could mean that as of the time of their certification, they were current. It could also be that they, like every other employer these days, are just happy that their employees turn out tested units and don't actively enforce that. I've bought a refurb from Amazon and it was not current so I got an older firmware version. It is more likely to be an older firmware version buying a refurb than buying a new tablet would be.
Toolbox will work either way, it will just work better with older firmware. Also, you'll want a tablet with FireOS 7 if you want to use the playstore because there have been changes in FireOS 8 that make Google Services incompatible and while Google Services may install, the Play Store will crash and so will other things that depend on them. You can stay away from FireOS 8 by not buying a Fire HD 10/10+ (2022) or Fire HD 7 (2021).
Edit: I'm sure the Fire HD 7 (2022) is FireOS 8. I'm not certain about the Fire HD 10/10+ (2021), but I think that's also FireOS 8.
So based on this, it sounds like the Fire HD 8 (2020) should work well with Fire Toolbox.