Thoughts on the series having played through all the games except Re: COM and the games that are just cutscenes in the collection:
I think overall the series went from strength to strength. The original games, while dated, still boasted strong production values. Given the timeframe of original release, the quality voice acting and tight action gameplay are really something commendable. More importantly, it holds up today. I think KHIII is a culmination and refinement not only of the mainline games but also the sub games in terms of combat. Ok, it's not Sekiro level tight, but what is? It's also less frustrating, and I say that as someone who platinumed Sekiro & Nioh and loves action RPGs.
Regarding the aforementioned production values, we cant just talk about graphics, although each entry is superb for it's time. The music, starting from the very opening menu, is just 10/10. You don't have to be an anime or JRPG fan to enjoy it - my wife has asked several times about the non Disney music and commented how nice it is. Even play it at bedtime occasionally, or the more upbeat stuff when driving.
The story is less dumb than people make it sound. I'm not sure what people expected from a game that is premised from the beginning on, essentially, multiverses, etc. Sure the story gets dumb crazy sometimes, but that's why I'm here! Buy a ticket, take the ride. I also think it's a fairly cohesive story overall if you pay attention and play or watch everything. It had a lot of twists and turns but in the end the good guys usually come out on top, which is no surprise given it's a Disney game. What I do like is some of the insights on lasting friendship and love, and hardship. It's more poignant to me at this point in my life than if I had played it as a younger person, that's for sure.
Finally, in just plain video game terms, these games are FUN! The action is tight, the story generally moves at a nice clip, theres plenty to do in terms of minigames, fighting, collectibles, and post game content in each entry. As I said earlier, one of the most important criteria is fun. The games always leave me with a smile on my face and in an age of grimdark games it's nice to have a game with kinda overbearing positivity, a game that even leans into it. I have no qualms about saying this playthrough helped me get over the death of our pet in December.
And I got all this content, hundreds of hours, for $33. Well, $63 if you count ReMind, which I haven't yet played. Still, great value for money IMO.
TLDR: The series gets a hearty recommendation from this ARPG/JRPG fan, give it a try if you're on the fence IMO, wait for the All In One physical disc for $50.