The Minutemen faction could have been so much better than it is. As you said, the only two actual Minutemen are Preston and Ronnie Shaw. I don't really consider Sturges, Marcy Long, and the other initial settlers to be Minutemen in quite the same sense as Preston or Ronnie. Sturges definitely is. You can read some about this on the computer terminals at the Gunner camp in the Quincy Ruins, which is where the Concord settlers were stationed for a while.
On the whole though, the Minutemen characters lack anything of real interest. Ronnie Shaw ceases to be a relevant character after completing Old Guns. After that she's basically just another settler. Compare this to the Brotherhood with Paladin Danse, Elder Maxson, Lancer Captain Kels, Proctor Teagan, Scribe Neriah, Proctor Quinlan, Paladin Brandis, Knight Rhys, Scribe Haylen, Proctor Ingram, and Knight-Captain Cade. These are all of the named Brotherhood members that I can recall off the top of my head. There are some others in various quests as well. These people have actual character. They have interesting personalities and dialog.
There are a bunch of named settlers within the Minutemen faction, but it's hard to even count them. The Abernathy Farm family is named, as well as the Finch Farm family, and the family at Warwick Homestead, among others. However, these characters lack any kind of personality. As far as the -actual- Minutemen are concerned, as in the Minutemen that arrive at the Castle and the ones that you occasionally see out on patrols...they just don't even have a personality. When I see some Brotherhood soldiers out and about, they at least have some kind of halfway interesting dialog. When I see a Minutemen patrol, half the time the only thing they say when I walk up to them and try to talk to them is 'huh?' and then they keep walking. Wow. Such respect for the freaking GENERAL of the Minutemen.
I'd love to see some more interesting characters for the Minutemen. You can recruit some various named NPCs that you encounter while exploring. Some of these at least have slightly more interesting dialog than the generic 'Settler' characters have. These named settlers also typically carry the bonus of being legendary vendors, so they at least serve a purpose. I'd love to see Minutemen quests that involve going out and seeking out a specific person to recruit. Instead of Preston constantly saying "I heard about another settlement that needs our help..." he could say "Ronnie and I were talking last night, and we got to talking about the old Minutemen member ____________ who managed to escape from __________ after the base fell to the Gunners and we have reason to believe that he could still be alive. He was the best sharpshooter we had, and he could be an asset to the Minutemen. We should see if we can track him down and get him to join back up" or something along those lines. And then this person could end up being an actual valuable Minutemen asset. A named NPC who isn't a legendary vendor, but rather a legendary soldier. I don't mean a companion, but rather a named member of the Minutemen who needs to be recruited. This would be a way to make the radiant Minutemen quests slightly more enjoyable and meaningful than going and clearing out the same group of Raiders from the same location time and time again.