I found a "workaround", the main reason for converting a imported model into morph is to be able to hide the unnecessary vertices without dealing with diving into the gdl code as @runxel suggested. Althought it could be great, dealing with texture mapping in the gdl code is a nightmare (sorry for the harsh word).
The workaround is to install the Archicad goodies specifically the 3ds import addon, with it you can have a much cleaner mesh representation in the elevations without the messy mesh looking.
Of course if you convert it to morph, the uv's will get lost
Collada import on the left and 3ds on the right in elevation view:
I am trying a similar workflow, and the Collada format seems to be the best one to maintain textures and geometry. The only drawback being that the object interface does not provide an easy way to remove the edges.
To remove them, open the object GDL script editor (cmd+alt+O), go to the 3D script and Find and Replace (cmd+F) "-1, -1, 0" by "-1, -1, 1" and select Replace All.
When you save the object, the edges will disappear!
Archicad 26 ARM, MacBook Pro M1 Max 64Gb, macOS Monterrey.