I'm really excited to play Re:Com as I've only played the GBA original. It's the easiest game to break and IIRC Reverse/Rebirth is more or less the most important single story in the series. It recontextualizes the original Kingdom Hearts and Sora's story in Chain of Memories.It has a significant impact on the later story bits in KH2 and 358/2 days. It also influences the plot of 3D through Riku Replica's appearance. It really is, in my mind, the start of KH's complex story fuckery that really goes off the deep end in KH2 (which I love but I can understand the arguments against it). Replaying KH1 I've forgotten how simple the story is while still being satisfying. There are basically only four "layers" to the story.
1. The layer of the individual Disney Worlds which follow the individual movies (with some exceptions. Atlantica looking at you) and are pretty simple to understand
2. The layer of Maleficent and the league of villains trying to kidnap the princesses of heart to open the final keyhole to.....take over the worlds I guess? I'm not really clear on her motivations. I cannot remember if it was to take over the worlds or destroy them or just because she's evil.
3. The layer of Sora looking for Riku and Kairi as well as Goofy and Donald trying to find the King
4. The layer of Ansem Seeker of Darkness operating and orchestrating everything behind the scenes explained in Ansem reports and explained in the very end.
The only place where KH1 gets complicated is when the layers intersect with each other or other games. It's no coincidence that Riku's story introduces the most complications when it's the intersection of 2, 3, and 4 (as well as Reverse/Rebirth). Ansem SOD's story involves not only 4 but the intersection of 4, KH2, BBS, and 0.2.