If you are not satisfied with Asus Keyboard or if you want to try a different Keyboard, just install any keyboard from the playstore & go into Settings => Language & Input => you can see installed keyboards select current keyboard which you need to use.
Option1 - Not working
Have already tried this and there no option to select or save or choose the current keyboard via Settings => Language & Input => installed keyboards.Always zenUI keyboard get listed first and other keyboards below and no option set the other keyboards as default
Option2 - Not working
if GBoard is used GBoard allows to change to other keyboards via a space bar but once zenUI keyboard is selected from GBoard space bar there no option via ZenUI keyboard space bar to go back to GBoard or other keyboards