I'm a casual player, myself, too. I don't mind the Pre-mades, but SoloQ isn't bad either. I can agree with you that there should be a way to separate SoloQ and premades, however, that probably wouldn't work due to premades not always being 5 people. Sometimes you have 3-man premades or even 4-man premades. Hell, my buddy and I are almost always duo together, so there's also that. This leads to the issue of the impossibility of a separate queue type, because they'd have to return it to 5-man premades like it was before. But, Riot changed that for the purpose of allowing groups of friends (or strangers, should you prefer) to play ranked together, even if they don't have a full team.
TL;DR: I agree that there should be a separate queue mode, but I don't think it'd be possible given that Riot wanted to make it easier for groups of friends to play ranked together, regardless of the number of people in their group(s).