Question I feel would work best here:
So the 1.5 + 2.5 Final Mix and 2.8 games just went up on Xbox and I was wondering if it was worth diving into the series as someone who has only played 3?
If you enjoyed 3 then go for it. The games closest to how 3 plays are KH1, KH2, and 0.2, but they all have their own unique mechanics. The rest of the games use the command deck system which focuses on customization, but sacrifices balance and fluidity. CoM is entirely it's own thing.
One last thing, all of the Remixes have had different bugs on different platforms. On PS3 1.5 had sound problems, and could sometimes load scrambled text in text boxes for conversations, subtitles, and menus. 2.5 also had sound problems, some loading issues, and a few broken hit boxes, but it most infamously exaggerated a rare bug from the original that would prevent a critical reaction command from appearing in the final battle. The PS4 versions of these games fixed very few of these problems, and created a huge number of new ones because they ran at higher frame rates than intended. This broke game physics, caused certain attack animations to play at double the speed, and broke tracking on several bosses attacks. These problems were
mostly fixed around a year after release with a couple of patches released back to back. The first fixed a bunch of issues, but created new ones, and the second fixed the problems caused by the first patch. If it weren't for the upcoming release of KH3 the PS4 version would've likely never been touched.
For these reasons I would recommend waiting just a bit to see how the xbox versions are. I've heard there's some new quality of life features not present in the PS4 version, which means Square has done some additional work, and hopefully some bug fixing. This is promising, but Square has a history of creating new problems in these exact games. But hey, third time's the charm right?