Do you shoot RDS with both eyes open? I have a pretty severe astigmatism in my dominant eye and don't have issue with red-dot blooms if I shoot both eyes open, otherwise I see lots of bloom.
If you still get bloom both eyes open - not a lot of optical options for shotguns. The one optic I kind of like in this realm is the Trijicon Reflex, the 1x24 with the ~13 MOA delta works okay for shotguns. A good friend has one mounted on Benelli M2, I find it very usable for practical work, it's not as easy to use with clays/smaller moving targets as a dot, but if it's a defensive gun, no worries.
I once had someone describe to me the purpose of a sling on a long-gun is equivalent to a holster for a handgun. It's the way you keep control of the gun when (not if) you have to move with it.
PS: To the OP. If I wanted a fight worthy shotgun, I'd just let my budget decide.
<$500. A Mossberg 590A1 with Ghost Rings; add Magpul furniture, a WML of your choice (I tend to favor Streamlight TLRs), and a sling. (I'd probably go with Mossberg #50774, personally. The addition of screw-in chokes is worth it to me).
<$1000. A Beretta 1301 Tac, with a +1 Nordic Tube, WML of choice, sling.
<$1500. A 1301 Comp 21", with a red-dot on top, a +3 Nordic extension, WML of choice, sling.
<$2000. Either two 1301 Tacs or a properly setup Benelli M4 (maybe an M2, actually).
$2000+ an actual magazine fed system (modified Saiga, 3-gun ready Akdal, etc).
Eventually, I'd spring for Aridus Industries' Q-DC shell carriers for mag-tube fed guns and if a 1301, a button shroud to avoid accidental shell spillage.