I have an 870 Express that came with a 28" barrel which is great for shooting clays. It's very accurate and holds a very nice pattern out to about 40-50 yards. I also have a 20" barrel for it for HD purposes. I bought it from Remington for about $120. You can find them cheaper except after certain elections.
Yesterday I bought a Chinese clone of an 870 known as a NEF Pardner Pump Protector. It came with a 18.5" barrel and a 5 round mag tube. You can drill out the dimples and make it hold 6 rounds plus one in the chamber. But that will void any warranty. These are well made shotguns. Marlin is the parent company for Remington and NEF and H&R (sister company to NEF). The only parts that won't interchange are the barrels and the chokes. Obviously Marlin wanted to make a cheaper version of the 870 using cheap Chinese labor. Some will take exception to that for sure. It is bad how they treat people in China. But half the stuff we buy comes from China these days. I paid $200 for a brand new NEF Protector with a 2 year warranty and a Parkerized finish which is better than the Express finish. I don't actually have any problems with my Express though because I take care of it. I also saw a Pardner pump with a 28" barrel and a 19" barrel being sold together for $240 just last week. That would solve about all of your needs but the place that was selling them is sold out. You might still find that combo around if you look.
I'm certainly not knocking the 870. They are excellent guns IMO. The clones of them are also excellent guns. If you check around you'll find lots of positive info on those guns. All the 870 accessories will fit the clones too (except the barrels obviously).
I bought another 870 type gun so my son would be able to shoot it while I was shooting one. Plus I think the value of the NEF guns are exceptionally high in the bang for the buck department. YMMV as always. I'm not trying to talk you into buying Chinese. I'm just giving you some information. You can make your own decision. You can find info about these guns on this web page
including info on the combo set. They list the combo as having a different set of barrels than what I saw though. They have a 22" rifled barrel instead of the 19" smooth bore barrel.
The thing about these guns is the price. You get a gun with a better finish and practically identical parts that will hold more rounds and will accept the mag extensions for the 870. You can buy a 7+1 Express for $650 or you can buy a combo with the two barrels you want and add an extension and have a 7 round tube mag for about $300. Plus you'll have a better finish.