The idea that He had to manifest in flesh and had to pay for the sins of mankind means he is subject, and not God.

You're missing the bigger picture here: the main goal of these rules (and the Holy Trinity) is for christians to be able to guilt trip themselves into giving Jesus' death so much more weight than they would be able to, otherwise.
"Humans killed Jesus, who is God's son and also God Himself, and Jesus' death on the cross saved us all as long as you repent and believe in His sacrifice" ---> this tale only has any weight if you believe Jesus truly is God's son and also God Himself (so you believe the "Holy Trinity")... because if not, them this was just some random judaean prophet's crucifixion by the romans thousands of years ago.

But imagine you're a christian who's up there in Golgotha watching the crucifixion of Jesus as it unfolds: the son of God, who you know is imbued with the very spirit of the one God Almighty Himself, is bleeding right before your very eyes and crying "My God, why have you forsaken me?"... as the minutes pass, he's there and he's dying and you're waiting and nothing happens. And he dies.
How do you even begin to backpedal or backtrack from this?... You don't. You double down. Yes he died... he died for me. His death saved us all. He expired all our sins. Now we'll be going to heaven. If we just believe his sacrifice.
In other words "everything that just transpired that would break my belief, I will instead twist and use to re-enforce my belief". You hold on to your belief and you strenghten it in whatever way you can. Or else your whole world fails.

Christianity does not work without the Holy Trinity and these rules that God Almighty seemingly "has to follow" for reasons unexplained. But that is just how mythology goes: it's a buy-in, and you either buy-in on the idiosyncrasies or you don't, based on how much it speaks to you. Thing is, this tale spoke to a whole lot of people. It still does. Why? Do people like the guilt trip? Or rather they like the feeling that "someone loves them so much, that this someone sacrificed himself to save them". I think we can see how that's a very powerful feeling, it's inspiring really - enough to make any miserable person feel loved, and perhaps even make them want to be better. Yes it's objectively a guilt trip, but this is one they feel inspired by. No matter how awful you feel (you are?), there's this big someone who did this one big thing for you, and he saved you. You just feel loved.

Ok but is it true or not?... The answer varies, but bottomline is, that's just how humans are. It's not any different in other religions. Again, that's mythology for ya. A collection of tales that speak to you, that inspire, that you hold on to, for some reason. If the myth / tale speaks to you, then you just shrug off the things that (yes) are illogical.