Ran into this exact worry on my own plug a few years back, a front clip off a two part foam buck. What I learned is the foam itself finishes its bulk expansion within a day or so, but the slow shrink and gas release keeps going for a good while after. The safe play is two to three weeks minimum before you commit to a mold. I gave mine about a month because I got busy with other stuff and honestly that worked out well, the surface settled and I had to re-sand a couple of spots that had gone slightly flat before I ever sealed anything.
The bigger deal than calendar time is temperture swings, and yours swinging cold nights to warm afternoons is exactly the kind of thing that moves a plug. Foam expands and contracts way more than plywood or the filler skin on top of it, so every daily cycle nudges the surface. If you can get it off the concrete floor and onto a bench or shelf in the shop, out of direct sun through any window, that alone kills most of the movement. A garage that stays closed during the day is usally fine even if the temp drifts, it's the big swings and sun load that hurt you. One thing people skip: seal the whole thing, not just the show surface. Foam is open enough that it breathes from the back side too, so get epoxy on the underside and edges. Thin coats, let each one kick before the next, and sand lightly between so you're not building orange peel into your seal coat. If your first coat looks like it's soaking in fast, that tells you the foam is still hungry and probly still moving, wait a few more days and see if the surface changes under it.
Your plan of epoxy, wax, then PVA is solid. Just do a final sanity check right before you gelcoat: run a straightedge or a short batten over the high spots one last time. If it reads dead true after sitting sealed for a week, the plug has stopped doing anything and you're safe to lay the mold.