It'll take a lot of trial and error to find the right ammo for your new barrel. If it does not, fall down on your knees and thank God that you were so lucky.
My advice is this: Start with the cheapest and work up.
Pick a distance. I picked 50 yards offhand.
Pick a group size. I picked a pie plate.
Pick a budget. I picked cheap
Yes, I'm kind of old school, but I also started out slug hunting as an outgrowth of bow hunting. All my stands were for bow. You will undoubtedly have different goals than mine, but the principle works the same.
Start with the cheapest round you can find that fits those parameters and go with it. I was lucky. I tried Remington Sluggers in my Rem 1100 smoothbore and got a 4" spread at 50 yards offhand and never thought about it again.
With my Mossy 500 rifled barrel, I tried a bunch of stuff and found that Brenneke 2 3/4" did the trick by putting slugs through the same hole at 50 yards offhand. I'd already tried 3 -inchers and they didn't group worth a poop in mine. Go figure.
Some folks spend $200+ a season on slugs, looking for the ultimate load for a 200 yard shot. Then they turn around and shoot ol' Mossy Horns at 40 yards. If you are looking to kill one inside 80 yards, I would not even bother with sabots.
Sabots are a whole 'nother animal. Each slug barrel is unique. Be ready to do a lot of experimentation. I've shot sabots out of muzzleloaders, rifles, and slug guns. By in large, it is hard to find a load that works. On the other hand, I shot 30-06 Accelerator (sabotted .223) for years out of Win Model 70 and called it my groudhog "Vaporizer." If you go down this road, make sure you buy a lot of whatever you finally end up with. I have seen a lot of lot-to-lot variation over the years in ammo. What works this year may not work next year, and minor variations get magnified when working with sabots.
One other thing: Two things will eventually come back to bite you on a Mossberg 835. One is the trigger-- nobody puts a decent trigger on a shotgun. The other is the way the barrel is attached to the rest of the gun. Eventually you will outstrip the shotgun's accuracy. Most of us live with this, because we don't buy dedicated slug shotguns. We also live in a world where most deer are taken inside 100 yards and it doesn't make a huge difference.