Macbook Air 10.13.6 - wrong resolution

[esuus]

Hey everyone,
for me the Driver version 4.3 using AirPlay mostly works for my 10.13.6 Mac, but no matter what I do, I don't get the correct resolution. Should be 1600x900, but best I get is 1440x900 which then shows two black bars on the sides. Every other resolution is blurry.

The Alt-key solution doesn't work, it showed various new resolutions, but not 1600x900.

What to do? Would an update to Mojave (10.14) solve this issue?

Sorry if this was asked already, I tried using the forum search and was surprised to find no results. Maybe it's this forum software that clearly comes straight from the year 1998.

Thanks for any advice!

Best,
David