To be honest, Preston and the Minutemen and Preston seem like panhandlers that have found an "easy mark". When they find someone generous enough to help them out, they keep coming back for more. It doesn't seem at all surprising that so many players dislike working for the Minutemen. It's ALWAYS "Thank you sooooo much! Now can you do us another favor right now?" Literally, if the Minutemen/Preston have their way, the Minuteman quests would be the ONLY things you would do in FO4. If the Minutemen and Preston were considerate enough to space out their "requests" -- a.k.a. demands -- it would change the vibe considerably. Of course, that would also require that Preston NOT appoint you General almost immediately.
It would have been great if at the end of the game they betray you for a twist, then it would have all made sense.
"The reward of good work is more work." Leads eventually to the Peter Principle: https://en.wikipedia.org/wiki/Peter_principle
As General, could you not assign Preston to some of his own quests?
Me - "Hey Preston I just saved that settlement of 12 people from that one Ghoul who lives halfway across the map, and in reality, had no idea that settlement existed."
Preston - "That's great news. I have another settlement that needs help. I'll put it in your map."
Me - "Actually Preston, looks like I just interrupted one of your naps - and it is 4pm. As General, I order you to take this task on. Should be easy."
Preston - [Blank Stare]
That's the big contradiction. As General, you have absolutely NO, ZIP, ZERO authority to issue orders to the Minutemen. (Beyond the same authority to tell Preston to "Go here" or be assigned to one of your settlements. Which is the same authority you have over ALL of your Companions.) You are the puppet and Preston is your puppeteer. 100% figurehead.