"Is FF14 an Isekai?" answered in order of expansion.
ARR - No, it is not an Isekai.
Heavensward - If Isekai is a metaphor for being somewhere that is not your home and feeling out of place, yes. Otherwise no.
Stormblood - Same as Heavensward.
Shadowbringers - Oh yeah. This is an Isekai.
Endwalker - FF14 is an Isekai and it has been since ARR.
A single-user space station out in the fediverse asteroid belt.