Besides that, Seidou and Akira have known each other much longer and their feelings have been developing much more during this time, Amon and Akira have known each other only for months…..

I’ve actually never once said that anon. I wrote this on the subject, in which I said that either could work narratively. It could honestly swing either way. I’m literally writing a Seiaki fic write now. I like both and honestly don’t care as long as the three of them are well.

That being said, here are some more of my opinion on the subject. Akiramon is set up in the first TG far more than Seiaki was. Akira tried to kiss Amon, not Seidou, and the drunk scene focused on them two, not on Seidou. Therefore, narratively, that build-up needed to be paid off for Seiaki to ever have a chance. Seiaki is better built-up in :re, though. But I remember preferring Akiramon until it actually became canon but without dealing with the Seiaki build-up. You have to deal with both ships’ build-up to get a proper resolution to all three of their arcs.

That being said, Akiramon is the couple that is canon right now. Seidou is happy Akira and Amon are together and doesn’t cling to them–he reminds me of Shuu dealing with Touka and Kaneki getting together. Akiramon has been associated with yin/yang visually (usually a symbol heavily associated with anima/animus, which often indicates a love interest) whereas thus far Seiaki has not. Akiramon also parallels Touken quite heavily, and Touken is certainly endgame. Akira and Amon often appear together in the calendar and in omakes. Seidou and Akira do not. (That being said, the calendar/omake argument is admittedly weak because Ayato and Hinami never appear together in the calendar and seldom do in the omakes, and same for Mutsuki and Urie–Urie and Saiko appear together a lot more, but according to what’s been set up in the manga itself, it seems like Mutsurie and Ayahina are likely to end up canon, or at least have their build up paid off.)

This isn’t me saying what I would like to happen. I genuinely enjoy both ships. It’s just me stating what are canonical facts. Also, Touka and Kaneki only knew each other for a few months too before a long separation and you really can’t judge what makes someone’s feelings “more true.” 

Now for Seiaki, I agree with you that Amon seems to project his focus on Mado onto Akira and see her as an extension of her father, whereas Seidou just sees her as Akira. That’s got to be dealt with. I think it’s very odd to set up Seiaki if nothing is going to come of it, which is why I still think the build-up for that ship needs to be paid off. The thing is, my faith in Ishida’s representation of LGBT people in TG isn’t very high so my hope for poly representation is even lower, so most likely one is going to be canon and one is not. 

I genuinely believe that narratively either ship works at this point. Akiramon needs to do a lot of work to escape Kureo’s shadow, though, but it’s not like Touka and Kaneki haven’t stagnated in this arc too. Seidou is almost certain to provide the development Akira and Amon need (and also Donato) as he basically represents what Akira’s father was actually like, whereas Amon is more what she wants to imagine her father was (though Amon is a lot like Takizawa and like her father actually was, he just doesn’t see it that way yet). I personally feel Seiaki is somewhat better written in :re, but we also haven’t seen a lot of Akiramon in :re, so there is that. 

I can see it ending two ways: Seidou outgrows his need to live for Akira and Amon being happy and finds belonging with his sister (and Kurona let me crack ship dream) while Akira and Amon help each other break away from their Kureo orbit, or Akira and Amon break up over whatever will happen with Donato and Seidou and Akira get together. 

Honestly? Both ways have the potential to be narratively satisfying imo.