Which (if any) of the Prime Frames Are Worth It?

Hi, I've largely been ignoring the Prime aspect of Warframe, since the vanilla frames have been working fine. I just stockpiled what blueprints I did get from the void, and didn't bother further. But using Frost, I was wondering whether to potato him or wait till I get Frost Prime.

A lot of the older posts on Warframe Reddit and other forums seem to say Prime frames are only slightly better, with slightly increased stats, one or two polarized mod slots, access to Void Energy Orb, and that Fashion Frame aspect, and not really "necessary" unless you get all BPs easily.

Also, Frost Prime seems to be vaulted now, which I think means I have to wait till he is unvaulted, whenever that might be, and then farm him?

And with the new SOTR relic overhaul, it seems you can only get one Prime part at a time, by closing fissures and using relics, no more endless Prime Part farming. Is my understanding correct?

Thanks in advance.