Take it from me bro. I was in same situation as you. Just buy the watch you like, they are not gonna upgrade the 15400 anymore as it's already got a clone movement. The only upgrade possible is Franken and that's expensive you can spend 1500$ on gen JLC movement.
I own a 15400 with a Miyota. I think that you are "overthinking" the snap date function a little too hard when you compare the whole package. Do you honestly plan to watch it snap everynight? Is it really that important?
I agree with you 1000% I will not buy 15500 as you said dials are total plastic but they are also .7mm thicker than the 15400! Totally stupid but so is gen so blame AP.
I suggest you get over the date snap and appreciate the crown winding, date and time setting, and movement finishing
I love my watch dearly. I would only want you to feel the same happiness I do when I wear it.