Just noticed today as I took the chain to clean it that my cassette has a fair amount of play. It / a Spedao 12s on a shimano free hub, about 700-800 miles on it, I don't see anything obvious. With the lock ring fully tighten, the play is still there.

https://youtube.com/shorts/wB4UrBzRaAU?si=QZE9XaxCSy3-bruV

Any suggestion welcome. Thinking of adding a spacer but all I can find is 1mm and probably too much

is it lateral play, or rotational play?

lateral play could be incorrect use of spacers (or lack of spacer), or incorrect lockring to secure the cassette to the hub.
rotational play is probably poor machining of either the cassette, the freehub body, or both.

I got the same problem recently.
CS-R701 cassette, maybe 3000km on a first wheelset, then upgraded to a new wheelset for 1000km until it developped the same play as you. i noticed just before leaving for the alps and the play was gone for 2 days before returning. a new SLX cassette on that hub had no play, the cassette also had play on 3 other wheelsets i tried. replacing the lockring did not help. i bought an identical r701 cassette and play is just gone now, so the cassette is definitely at fault. there is no obvious damage to the cassette or dent to the back of the spider. still a mystery to me

do you have enough compatible parts like me to troubleshoot and confirm the cassette is at fault?

Unfortunately no, I don't have spare wheelset or cassette. I will check with friends

As for spacers, I don't have any installed, could it be my problem? I didn't think I needed one on a 12S cassette with shimano free hub

Yes I Will ask a friend to swap stuff around

Actually you do need a 1.85mm spacer for a 12 speed MTB cassette on a 12 speed HG ROAD freehub.

Ah ok then this would explain it lol thanks.
I don't see 1.85mm allowing for the lock ring to engage. I will buy a kit withb1 and 1.85, thanks

Well the spacer did the trick, I did 1mm, minimal to no okay, went up to 1.5mm, seemed even better so it stay with that