So maybe to still grab that feature it might be possible to add a (large) capacitor to keep the device going for 2-5 seconds?

On screen resolution, please go with fhd 1920x1080.
I own moto z as you know and the 2k screen is useless apart from showing off, viewing photos and playing games maybe but obviously drains the battery more in any situation due to more effort to render up to 4x more pixels than fhd depending on actual resolution.
Also in arm length distance there is absolutely no difference to make out imo.