I know we want to make a big deal of this, but the sales are in line with sequels in the Final Fantasy world. FF X-2 sold half as much as X. FF13 -2 sold half as much as 13 and Lightning Returns sold half as much as 13 -2. Sequels to Final Fantasy games have never had amazing success.
We also have to consider that Remake released on a console with over 100mil users on it. PS5 has half of that. No one should be expecting each part to be doing 5mill+.