Ok so sales are what they are, the question is WHY do you think they are what they are now?
The main culprit is not any of the your points, although sequels in FF historically do decently worse than the first one.
The real issue is the pretty abysmal software sales to console sales during this PS5 generation in Japan.
We're seeing a lot of PS5 being sold but the software sales are not there if you compare to the PS4.
And since FF games are only on PS5, there's not real recourse to sell more in the region moving forward.
They're just going to have find a way to get sales elsewhere if they're not changing anything in their release strategy.
I don't think the issue is user base.
Switch release could do what? Increase to 500k, 600k first week? That is still too low for a FF in Japan.
Xbox lol with these 2k units old?
I think it would be significantly more tbh, if such a thing was possible.
Just look at the most recent DQ game released last December.
01./00. [NSW] Dragon Quest Monsters: The Dark Prince # <RPG> (Square Enix) {2023.12.01} (¥6.980) - 346.583 / NEW <80-100%>
This is not even a mainline game but a spin-off DQ series.
The game was even supply constrained due to Square Enix underestimated the demand(something that is pretty common for Square Enix to do) so a higher than usual digital ratio for the Switch.
SE made the right move taking the EGS deal, and they should do it again. Epic will pay them a lot more than Steam sales will bring in 1st year anyway. I don't even think FF7Remake has sold 1m on Steam yet.
Epic Games has been moving away and being more selective about doing these exclusivity deals because they've realized it's not bringing engagement to their store like they're hoped it would.
I'm not sure Rebirth would get a deal or it would be a much reduced pricetag that might not be worth it.