AshX Scalling performance lag has been improved in latest Mac os versions, at least I did not see any mouse lag.
For non portable, I have used QHD (2k) resolution on 27 and it was very good, no scalling needed, just native resolution. I would say 32 QHD (2k) would be even better running native without scalling, slightly larger letters. These are best, but somehow people make mistake and buy 4k.
For portables, I would put in top list this one:
ASUS ZenScreen MB16QHG (Did not test it yet myself, but signals about it on Amazon reviews, Asus support, and overall specs are good. This should be flicker free device). Plus it has good mounting design, meaning you can put it on small tripod without adaptors and it would sit stable. So you fix posture problem at the same time. I am looking for genuine feedback on this specific from ledstrain people, but on Amazon I found few comments where people say - they are happy with it because it solved eye stran problem. Others complain about stuff I do not care personally.
I would avoid anything:
OLED
Arzopa (it is utilising PWM)
and many others - as there are lot of small solopreneurs selling their own portable bundles with stands, but its all cheap chinese imports IMO. They can not compete in any reasonable sense with big companies who know ins and outs of Asian technology suppliers/manufacturers.