Like many I don't find this sort of numbering useful or edifying. At it's core this is clearly "2nd edition 5e D&D". Putting in software version numbers doesn't really add much to our understanding of that, at least not when D&D has such a history of haphazard use of "edition" numbering for things that were sometimes new editions of existing games and were sometimes sequel games. That said, since you defined your numbers as clearly as is feasible, I'll accept your scale for the purpose of this discussion.
It's really hard to say based on the playtest because it centers on races and backgrounds, the first of which we knew was the thing they were most keen to change, and the latter of which is changed partly to accommodate the first (moving the ASIs from race to background) and partly to bring it in line with how backgrounds are currently being done (feats in place of background features is what they've done for Strixhaven and Spelljammer backgrounds, as well as the Dragonlance UA). They also seemed to want to announce far-reaching mechanics tweaks (the changes to inspiration, crits, and "D20 tests") here at the beginning.
I'd probably rate what we see in this first playtest a 5.3 on your scale, but I won't be shocked if they are more conservative in the changes in future playtest packets, which might bring it down to a 5.2. If there are more far reaching changes they haven't sprung on us yet it might be more like a 5.4 or even 5.5. It would require a radical rethinking of the materials presented so far for it to be 5.1 or 6.
