Sony confirms that owners of Spider-Man for PS4 can't upgrade to Spider-Man: Remastered on PS5 and also cannot buy it seperately as it is part of the Miles Morales Ultimate Edition

https://kotaku.com/sony-confirms-theres-no-free-ps5-upgrade-for-ps4-spider-1845154011

If you buy Miles Morales for PS4 however, you can either freely upgrade to the PS5 version or pay 20 bucks extra to get the Ultimate Edition which has Spider-Man: Remastered

FWIW, your PS4 version will still work on PS5 (without upgrades of course)

I think this isn't complicated if you realize that Spider-Man: Remastered is being treated as a seperate game

MM PS4 -> MM PS5 for free

$70 -> MM PS5 + SMR PS5
or
MM PS4 + $20 -> MM PS5 + SMR PS5

SM PS4 -> SM PS4 on PS5 with BC

I think I just opened some kind of black hole in my room