Just to check, did you get Sȳroti from the table here: https://wiki.dothraki.org/High_Valyrian_Adjectives#S.C8.B3z-type
I ask because I've made the probable error of clicking down to that table a few times and not noticing the blurb where it says "A small subset of type II adjectives, whose stems end in r, have an unexpected lun/sol.nom/acc.sing. ending in z. ... Aside from this anomaly, they are entirely regular type II adjectives, so, postpositively:" [my emphasis].
Which means I then have to scroll back up the page for the prepositive table: https://wiki.dothraki.org/High_Valyrian_Adjectives#Prepositive_2
Presuming the blurb "entirely regular type II" means sȳz follows the same pattern as kirine in the prepositive, the lunar plural genitive is: sȳro(t). The (-t) is for when there is a following vowel, so here it would just be sȳro.
That is how I am now looking at it anyway.