I am right there with you on the aimpoints absurd price
so get a Aimpoint duty RDS, you can find them for like $350-400 on tacswap relatively new, with a mil/leo discount or other discount you can probably get one for $450 new
thats what I am likely going to do, Eotech feels goofy on the M4 but they are great optics I got 3-4 of them.
besides that aimpoint acro p2, Trijicon RCR, Steiner enclosed red dot, Holosun 509T's are fine as well but would not be my first or second choice