An overall ramble but you get the point. When it comes to legendary groups and designing them Game Freak ultimately is more concerned about thematic cohesion and making all the trios fit with each other thematically and creating a general "throughline" with how their stats are designed, and sometimes their types are simply meant to be thematic. They are not concerned at all about how well they perform against each other or how they perform relative to each other in the competitive sphere, especially since most of the major legendaries aren't even allowed in VGC for the most part while most minor legendaries, particularly earlier ones, weren't designed with the intent that they would do well in PvP.
Fully agree with what you said, though i'd add a "small" niptick.
I think recently they've started to care a bit more about how the plot relevant legendaries (mainly cover, but not only these) do in competitive.
(disclaimer: I am excluding UBs and Paradoxes from the legendary group, though some of these definitely still apply to this concept)
Ultimately, they do need to sell their games, and one of the way to get people to buy the game is making them look forward to use their fancy new legendary tool against other people. All of gen 7 8 and 9 legendaries were made with this in mind, combined with thematic. Ofc not all of them turned out perfectly, but outside of Zamazenta (which I swear would probably have actually seen play if it had Body Press last gen, idk why they didn't give it to him) and DawnWings (which sadly was just a worse Lunala after all) all of these legendaries saw play at highest level in VGC.
Obviously some saw more play than others, but bar certain outliers (cough cough Incineroar) there was a pretty healthy variation of archetypes and restricted representation last year, and even right now despite the omnipresence of Palance and Flutter Mane you do see quite varied strategies being used, with a decent chunk of off-meta picks to counter whatever is meta now. Except Wo Chien sadge.
To add to that, pretty much every legendary has been basically minmaxed on some front in last three generations. Zamazenta's minmaxing was a pretty sad case of "pointless", but all the others had their niche, from the 4 tapus which all had strategies centered around them (with Tapu Bulu being the less valuable but still had some use), so did all gen 8 ones, and right now 3 out of 4 ruin legendaries are seeing widespread usage since who would have guessed, minmaxed stat spreads backed by numbers are good.
Heck Zacian managed to be the 2nd ever pokemon banned to AG (twice tecnically lol), and while not available just yet, I'm quite confident some people are considering Miraidon and Koraidon very close to prenerf Zacian-tier of power, ironically kept in check by the very same threats they enable since their own field conditions can be used against them.
They've also shown that they are willing to balance them out, sadly (or maybe luckily) their only balance attempts happen between games, the only type of mid-generation balance is done through additional movesets / items or new pokemon that support them added via 2nd version / DLCs, but the changes to Zacian / Zamazenta for example are a good indicator of how their design philosophy swapped from "all we care is thematic" to "we have a thematic, how do we make it work competitively" over the last few generations. Most likely cause they are *really* trying to push the games to e-sports status.
VGCs is also to some degree a example of very centralizing metagame that however still manages to be varied enough to be fun to play (most of the time, we do not talk about CHALK), since it basically cycles "op" and "counter what's op" and "counter what counters what's op", which in itself keeps the metagame a bit fresher.