I set up a youth model 870 for my daughter to shoot and you can see my results in the pattern section. It is a 40 yard killer with the HW#7 and a Trulock choke.
The 20 g Shurshot stocks are gonna be pretty hard to find from my reading, because that was one of my options when I just started researching a 20 for me. One place online says they have them, but when you click on the link it says backordered. Most every other site I found says unavailable. Someone on here recently found one though.
I had a similar list as you and wound up with a Weatherby SA-459, I picked it up Saturday. I liked the way if felt in my hands and it really does not need a bunch of mods. I just ordered a Trulock for it and will post pattern pictures when I get done shooting. If it is too tight, I may also use a Carlsons for when I am hunting in the swamp for a little more open pattern. FL-Boss gives the gun very good reviews.
I looked at the SA-20 Mossberg too. I have never owned a Semi before, everything is either pump or O/U.
Not sure if I am going to like the peep sight, but I will shoot it plenty before opener to be sure I am comfortable with it. If I don't like it, it should be easy enough to change. I own a red dot, but really prefer open sight shooting. It has a rail though if you are in to that.
I also held an Escort in my hand.
But since I wanted a pistol or thumbhole, I figured to just get one already on the gun. Out the door for the gun and sling, with a 10% discount from BPS for getting a credit card would be less than $600. I was at 630 and I bought 2 boxes of Fed HW and one box of Mag Blend. Since you are not ready to buy right now, after hunting it this season, I will give you an honest review after the season.