I grew seed of seryu a couple of years ago and got mostly strong growing seedlings with normal Japanese maple leaves but 1 with deeply cut leaves and cascade habit so not surprised you have variability.
Where you cut depends on what you want to create and , with trunks this size, where the roots are. Personally I'd cut low to get taper into the trunks. Hard to tell in photos but possibly no 1 above first (big) branch? no 2 either at first branch or at the upright branch at rear?? If you don't mind trees with little taper then cut higher.
I have done some of mine already. I prefer to prune them early. They tend to 'bleed' profusely if cut near budburst but cutting the roots seems to stop that almost instantly. I'd cut back and root prune at the same time - for me, end of July or Aug, for you maybe a little earlier? Can be done now but do not keep it too wet after. It is always better to see what the roots are really like so you can make a better choice of where to cut anyway - shape of trunk, possible front, any branches, etc.
Looking it this Maple it has some challenges in store for me, it may not make a great piece of work but time will tell. Any pointers or slaps on the wrist are welcome
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