SS expanded movesets weren't just "a couple new moves" but rather lite fighting game movesets that redefined the way you engage with the core mechanics. The simpler copy abilities in Adventure and DL2~3 are sort of situational and the pacing is completely different as you are forced to work with more limitations in these games. Also that kind of moveset was very rare in SNES platformers, most of then having one or two moves, so 20+ abilities with 4~8 moves each is no small feat. Add on top of that its very unique minigame structure and great co-op feature (maybe after wii era this doesn't seem like a big thing, but I can barely think of snes coop platfomers, let alone that allowed it seamlessly) and yes you have a very ambitious game. I really think you underestimate the amount of work and thought that went into this title.
And yes Kirby 64 may not be as ambitious as those two, but again the ability to combine any of its copy abilities to make a new one is not just a gimmick, but a core mechanic (and one that goes in opposite direction of the fast paced SS, that sadly will probably never be revisited) and the use of klonoa inspired 2.5D, in which every stage is completely unique, still makes it a standout in the series.
I just fundamentally disagree that expanded copy movesets "redefines core mechanics". Nothing about the combat or platforming comes close to necessitating or even really encouraging the use of those mechanics. They're fun, they're good to have, but it feels more like general iteration and QoL than it does redefinition.
Of course, some copy abilities do feel nicer than others thanks to expanded moves (Sword is always the obvious example) but I wouldn't say it transforms or changes the way the game is played on any deep level. When it comes to higher level play there are some cool tricks esp vs bosses, but even then its not super duper intriguing and I reckon most players beat the game without really diving in all that deep into the (imo not that deep) movesets.
Most of the minigames don't really redefine Kirby, they're just fun asides. The sub-games are just neat little distractions, the arena is a boss rush mode and Gourmet Race is just the platforming with a greater emphasis on precision rather than the series' basic combat.
Is SS a beefy package? Yea absolutely! But the core gameplay still hadn't changed much and the minicampaigns basically combine to rival the length of a single adventure without really feeling that distinct from one another. Great Cave is probably the closest it gets to really breaking the franchise's mold on a structural level.
Idk maybe I sound crazy but I really dont view SS as anything but a solid, largely iterative sequel that sacrificed one long campaign for a bunch of slightly different campaigns and some neat minigames. One of the SNES' best, but not because of immense ambition and moreso because it further refined the core campaign and threw some fun extras to compliment the campaign.
Maybe I'm also just a bit of a SS underappreciator. Love that game but it probably wouldn't crack my top 5 Kirbys tbh. I remember my friends in high school getting me so hype to play it when at that time I had only played Nightmare in Dreamland. I really loved SS but then when I went on a franchise binge over the next couple years I struggled to really understand why everyone online talked about it as the best Kirby game. Maybe if I was around when it had originally released I'd hold more appreciation for it contextually, but I'd take KA3 as my SNES Kirby of choice most days.
As for K64, again I don't think copy mixing really redefines or reinvents the franchise in any huge way. Its awesome to experiment with what cool new stuff they added, but it pnce again feels like iteration more than anything. Its also probably my favorite Kirby game aesthetically but idk if I'd say that because it takes advantage of new hardware its automatically super ambitious or anything.
Again, if just that makes a game ambitious, what game isn't at that point?