I would agree the ISO needs to be updated more frequently then it currently is.
While I agree with @Neumie that features such as sound are not important, it should still probably be kept in mind when releasing an ISO.
Normally when checking hardware compatibility (or debugging hardware issues), I generally suggest downloading the current ISO and testing the Live Environment. But that's not really true for Solus, we've just cross over the year mark since Solus 4.3 was released and there's no sign of 4.4 that I'm aware of.
My thoughts on the ISO issue are two fold:
Maybe have only one ISO that is constantly updated (bi-yearly/quarterly/whatever make sense), and have the others update on the point releases 4.4, 4.5...
Personally I wouldn't care which ISO was chosen. As long as I know my hardware will work, I'll spend the time needed to get my DE of choice installed
How difficult would it be to build/update "minimal" ISO that has an Installer and little to nothing else?
I'm thinking should be possible to make the Installer able to install any Desktop Environment, as well as connecting to the repositories to download and install a fully up to date OS.
While it would be nice to be able to do minimal hardware testing testing of hardware (sound, display...), I question the feasibility of this.