A few thoughts:
1. I prefer 20 ga to 12 ga, very much. But most of the good HD shotguns are 12. So I looked high and low, and found what I wanted in 20 ga. I have a Weatherby SA-20 semi-auto. It comes in a turkey version and a defensive version. I love it (I got the defensive version). They are hard to find. Mossberg makes one just like it, in their Mossberg International section. It is made in the same factory in Turkey as the Weatherby. They are good shotguns. I have mine set up with a red dot sight for HD work, and the included iron sights (came with the shotgun) are still in place for co-witness and back up use.
2. I am not handy with tools at all, not one bit. I am not very mechanically-inclined. However, even I can change a shotgun barrel in about 100 seconds. They are very easy to change. So, you can get whatever shotgun you want, and then get the spare barrel for the other purposes. Get an 18 inch HD barrel, and then get a longer turkey barrel. They are very easy to change out.
3. The terminology for slugs can be confusing. "Rifled slugs" have grooves cut on the side of them to make them spin as they fly, simulating an effect of being shot from a "rifled barrel"...BUT rifled slugs are intended to be shot from SMOOTH bore shotguns. So, you can get a smooth bore like a 20 gauge Mossberg or Remington, or the Weahthby mentioned above, and use rifled slugs in them, quite well. Sabot slugs and Breneke slugs (spelling???) are intended to be shot from rifled-barrel shotguns. I am not sure, but I don't think you can shoot rifled slugs from a rifled barrel. I know that sounds backwards, like you should be able to shoot rifled slugs from a rifled barrel, but I think you can't.
4. I use 3 inch magnum #2 Buck shotgun shells and rifled slugs for my self-defense loads. Some people use #3 Buck 2 3/4 inch shells for their 20 ga self defense. In 12 ga they can use 00 buck, or #1 Buck, but those do not exist for 20 ga except in specialty loadings that are expensive. It has been explained to me that when you try to pack the shot for #1 Buck or 00 Buck into a 20 ga shell, the only way to pack the shot in there will also distort the flight of the pellets enough to render the whole thing useless, and that is, supposedly, the reason that major ammo manufacturers do not load those sizes of shot in 20 ga shells. I have seen some specialty places that claim to be able to load 20 ga with #1 Buck, but they are expensive and I'm sure that if they were able to be loaded, the major guys would do it and sell them mass market. I think some handloaders do load the #1 and 00 Buck in 20 ga shells, but who knows if that really flies well.
5. I also have a Mossberg 20 ga (model 500, persuader) and I had a picatinny rail mounted over the reviver for a red dot sight. I was able to find a metal forend that can be put in place of the stock plastic one, and the metal one can rails mounted to it, so I now have a light on the front of my Mossberg.
6. I always thought that pump was "more reliable" than semi-auto for self-defense shotgun, and I always though pump was a better choice for stressful home defense. Then when I was practicing with my Mossberg, I found out quickly that it is VERY EASY to jam up that pump shotgun by accidentally short stroking the pump and when it jams up, you are dead. I knew that it was possible to do that, but did not realize how EASY it is to do it accidentally even when one is trying really hard not to short stroke the pumping. I was doing some realistic practice, simulating a real stressful home defense situation and that darn thing jammed up on me after just one shot fired. So, after that experience, I bought the Weatherby semi-auto. That thing seems to be much more reliable than the Mossberg 500 pump.