I've shot 8-10 birds with 20's and have a few set up that shoot very good. I broke my collar bone last month so I will be for sure using a 20 and shooting from the right, I'm a lefty. Funny thing is I told my dad just before it happen that I planned on this being a 20 only year. Done it for a few seasons but always gravitate back to my benelli M2 since it shoot awesome.
This years gun will be a 870 with SSX shooting hevi 7's. good patterns to 40 yards.
A red dot will make the transition easier, especially the dominant eye issue. Good luck!
I shoot with both eyes open but I put a sticker right on my gun reminding me to close my left eye shooting right handed.