From memory you need to set the mobilescalefactor for each android profile. You do this in the DefaultDeviceProfiles.ini file located in the config folder. I attached mine (zipped) DefaultDeviceProfiles.zipif you want to try it out (you should be able to just place it there I think). Noticed in several places I needed to set:
r.MobileContentScaleFactor=0
&
r.DefaultFeature.AntiAliasing=3
to get it looking right on all devices.
RE: all the issues with 4.20+ make sure to report it to Epic if you haven’t already - the more voices reporting it the more likely they might fix it one day 
https://epicsupport.force.com/unrealengine/s/