These are valid points, I'm currently going through the dance of building a custom cross platform gui for mobilde+desktop tailored to my application. Proper localized text rendering is hard, Harfbuzz and Freetype are solid options to start with and are available on recent Linux desktops. I've abstracted the text rendering to replace Harfbuzz and Freetype with platform native libraries on Apple and Windows.

The renderer itself is abstracted and defaults to opengl 3.3 with a fallback to software rendering as last resort and to be portable.

Long story short: It's not easy but very possible to build a polished cross-platform GUI smaller than 1MB. My app binary currently compiles down to 135 KB on desktop, in the end I aim for less than 2 MB for the whole application.

However I wouldn't recommend this for most applications where a framework would be a faster option for a traditional GUI.