I like your thoughts on delegating responsibilities for specific things. I don't know your group, but my best guess is maybe they still don't feel comfortable in taking charge of the game - or at least sharing an active role in the process. Something that may be helpful is getting everyone used to the idea by having a few sessions where you have a "revolving" GM. One person leads for a short time(say, 15-20 minutes), then passes all the responsibility to another. Another idea: Have some kind of oracle like Mythic. Unfortunately with this, you may have the same issue you are currently having, where one person takes the lead on calling for checks and rolls. In my opinion, you are a bit ahead of the game with having some mashup of PBtA in your house rules. From what I've seen on actual plays, these games do a lot to encourage a collaborative experience. Chances are, you are probably going to have to make peace with someone being in a moderator role - at least for a little while. Maybe after a few sessions of a "musical chairs" style of DMing, your group could switch to one person leading each session. After awhile, maybe it will become easier for your group to share responsibility.