Curate, connect, and discover
I've had a similiar interpretation and I saw someone in the tags say they've also seen soulmates in a way as "mission for eachother/together" and I actually completely agree with that too, soulmates is a broad concept that can fit into anything, good or the bad. There's this other media im in where these characters are connected to be soulmates but the other is extremely toxic and abusive. So soulmates in their concept doesn't mean good, that they're just connected by souls fortunate or unfortunate. That would make sense for Saiki and Aiura, and fits more as shown that he needed her, because his future wouldn't have changed if not for Aiura..? In his case, being somewhat fortunate.
I wanted to add that when someone's souls are connected, they're the one who impacts their future the most, for both or for one soul. That's what a soulmate generally means and stands for. Whether it's romantic or not is what happens between your soulmate, it's not necessarily all romantic, just that your souls are connected. Aiura's soulmate is Saiki, but we don't know who Saiki's soulmate is if I recall, it's never really been said since she (miko) doesn't mention more about soulmates.
the idea of soulmates or "fated partners" is intriguing to me, heres my theory abt the soulmate mechanics of saiki k ^^
my 1st and main theory is that its related to mikos ability to see compatibility, ppl with the highest or maybe only if its 100% compatibility r considered soulmates. however tho i do believe this in general bc it has the most canon backing i dont actually think this could be the case for saiki and aiura, we never get to see it but i think if wed been shown the compatibility (or chance of success) between these 2 it would be low (u can reference my miko analysis for y i think this) so i think theres more than 1 way for ppl to be fated partners. the idea that this is the only way to be soulmates also contradicts info ill provide below
another theory ppl have had is that ur soulmate is literally ur future partner, and at 1st i thought this was implied when i read the manga bc miko says she was "looking into her own future" when she found her soulmate, however after checking a few translations, this seems to be a mistranslation, she uses the same language as she does for any future or non-future readings. this post would b rlly long if i went into it but i think this theory is completely off the table in saiki and mikos case, it would contradict what we know (abt mikos powers, future not being stagnant, the fact she went out of her way to change the future to find her soulmate which would imply it wasnt in her future b4, and more), but maybe the future isnt entirely off the table as a factor for soulmates in general. i do think its possible considering all her talk of future and fate
canon facts abt soulmates:
-they r hardly mentioned at all. the only mentions of soulmates r: 1) when shes talking abt saiki. 2) chiyo thinks shes shuns soulmate, miko doesnt believe so. 3) in 236, a client asks miko if hell ever get married, miko refers him to "the woman of his dreams", NOT his "fated partner" or "soulmate." its unclear if this woman is his soulmate or if she foresaw something else (im in the middle of a reread so pls correct me if im wrong so i can add to this and on that note lmk if my translations r ever off)
-miko mentions she doesnt believe chiyo is shuns soulmate, but she didnt scry this and her language implies she doesnt know for sure. this means their 2-4% compatibility doesnt actually 100% ensure that they arent soulmates, but it might be what made her think they probably arent. this means compatibility may be linked but not entirely indicative of being soulmates
-miko mentions that she was "rlly feeling it" the day she found her soulmate which may imply its very hard and rare for her to see. she also makes no attempts to get any1 else with their soulmates iirc so it seems unimportant and not guaranteed, its only a desire
-compatibility, which may be linked to soulmates, is also referred to as "chance of success", which may or may not be an interchangeable term
so with all that in mind heres my theories for all the ways ppl can be considered fated partners:
-highest/100% compatibility
-your ideal/dream person (i think this 1 may be possible to be unreciprocated or possibly u have to be each others dream person with ur ideals, morals, and plans for the future lining up)
-the person u get with in several possible timelines or in the foreseeable future
-the best future that can be foreseen is 1 where u r together
i also dont think miko can conciously choose or see which "type" of soulmate shes seeing
we also have to consider that compatibility and the future BOTH canonically change pretty easily and altho that concept interests me in the context of soulmates, its rlly hard to consider that here in a way that doesnt stretch too far from theories into just fanon and where soulmates would still mean anything at all, so ill table that for now but the idea that soulmates can be changed is very intriguing and i think its pretty likely when i consider how all of mikos seen powers work
now specifically for saiki and miko: i think theyre some form of the last 1. i think fate brings them together bc every future where they dont meet is a bad future, at least for saiki. this is bc i believe kusuo could never have stopped the volcanic eruption, or at least that it wouldve taken him many more years to do so without miko. therefore each future where theyre happiest is 1 where theyre connected in some way