Not a direct answer to your inquiry, but have you tried the new plasma-login-manager? It’s a replacement for sddm and it’s maintained by KDE themselves.
I don’t know exactly about the sddm but I had similar issue woth the plymouth theme I was trying and I just had to manually swap out the image with required resolution one…
For sddm and plasma splashscreen I just used different themes and mixed up some elements to make it look as I wanted… PLM is too plain for my current setup
Apart from making your friends who use windows question their choices and thought of calling themselves ‘tech enthusiasts’…
Always nice to tease windows users… I’m not gonna miss the opportunity on that one… That’s why I use sddm sugar candy theme
Every small little details I can make it look unique on my PC is always nice, 100 small details make one massive statement ‘freedom of choices’…