How do I get the scaling options for "larger text - more space" for an external display?
I had the same issue as you on an LG monitor an I think I found a solution to get the Larger Text - More Space option back. 

When I got my LG 43ud79-b, everything worked fine. The mac recognized the monitor with 4k res and I was able to go in to my monitor settings and select the more space option.

At one point the monitor had a suggestion overlay saying turn on HDMI Ultra HD Deep Color by going to Picture -> Picture Adjust -> HDMI Ultra HD Deep Color to on, which I did. 

After that point, the monitor was only recognized as a 1920*1080 monitor and the scaled options only had resolutions instead of the Lager Text - More Space options.

Turning off the HDMI Ultra HD Deep Color option makes the mac recognize the monitor as a 4k monitor again and brings back the original scaled option. Hope that works for you.