I am in the rather niche group of owners with 4 kids, an OG 2013 P85 S with the rear-facing trunk seats and a 2017 X 6-seater (which replaced a Sienna).
While I cannot comment on the Y 3rd row seats, and while it is possible that Tesla will pull something amazing out of the existing vehicle frame, I would recommend you go for the X, as noted by the above posters. Cargo capacity will indeed be reduced in a 3-row Y.
In case Tesla go with rear-facing seats again on the Y, a few observations from personal experience.
The S trunk child seats were amazing in concept, but in practice they had two main flaws:
(1) There was little to no ventilation for the children sitting there. Even with a heavily tinted trunk window they could get very hot in the summer (cold in winter was not an issue since they are usually dressed to the gills to begin with).
(2) Some children did not tolerate the rear-facing / forward acceleration and got motion sickness. Even in children not prone to this, the poor ventilation often exacerbated it.
- They were also not very comfortable on long trips (impossible to move / stretch), but were fine for short to medium ones.
Obviously now that the kids are older the S again has a cavernous flat trunk, and the X is the family mover, for which it excels.
If you are not in a huge rush you could certainly wait and see what happens with the Y, or get an X now and later trade it in for a Y.
Either way, good luck with your purchase, and stay safe.