Can't add to your list. Though having two grinders for the wire & cut-off functions is definitely the way to go (I have only one).
Instead I wonder: do you have any problems with the MIG & burning holes in tubes? What kind of settings are you using? I have a Hobart Handler 125 or 135, whatever it is its pretty similar to your millermatic I think.
I ask because I'm in the process of experimenting with a trailer & am debating the merits of a tallbike project.