I find this limitation rather artificial and annoying

It is not artificial. This is how Windows’ native font API works,
and for a good reason - many fonts are manually hinted to
raster grid, so trying to show them up at arbitrary size (especially
by small sizes) - you’ll just get some blurry crappy
quality. Of course this depends on rasterizer, but it seems Scintilla
is dependent on Windows’ APIs. Manually hinted fonts
(e.g. Times New Roman, Verdana) give significantly better quality so this makes sense.

especially with raster- and raster-based fonts, which work only at certain sizes.

This is quite self-contradictive: raster fonts are by nature non-scalable and are created initially in specific fixed sizes - each size is a separate
image stack.

you might get a better chance if you address this feature request at github.

Cheers
Claudia