Lots of options - that's part of the appeal.  I've got a factory .357 Mag barrel that I had cut back to 6" and threaded.  I've also got a threaded 16" .357 Max from MGM.  Both are silly quiet when shooting subsonic suppressed.  I've also got a couple 14" .357 Mag barrels.  Shooting from a rest, any of them are as accurate as a rifle.  Lots of fun.

I think most barrels will require you to move the front sight back if you have them threaded.  But it's not terribly difficult, and you can have it done when the barrel is being threaded.  Or as Alaskanfire suggests, run an optic and don't worry about the front sight.

Factory barrels with a .224" bore all tend to be fairly slow twist affairs.  I have a 12" .221 Fireball and a 23" .223 Rem, and both of them do best with bullets in the 35-45 gr. range.  If you're shooting supersonic and poking paper or blasting steel, it doesn't matter much anyway.  Shoot whatever the gun likes, and don't worry that you're not shooting the heaviest bullet out there.  Take them after jackrabbits or prarie dogs and they work marvelously.  If you want heavier bullets, one of the custom shops can make a faster twist barrel for you.

My first .44 barrel was only so-so, but the one I have now shoots great. So it's hard to say.  And while a .45/.410 barrel is one that any Contender owner should have, I have also found they don't do either cartridge all that well.  The .45 shoots patterns, and the .410 shoots doughnuts.  And I've got the longer 16" barrel.  I can't imagine the shorter barrels would do any better.  They're kind of fun nonetheless.  But you may be better served by getting dedicated barrels for each cartridge and skip the combo version.

.30-30 isn't bad,  but there are other cartridges that are more efficient in the shorter barrel.  I had one for a while, but I figured anything I would use it for could be done as well with my 10" SSK barrel in .300 Whisper.

The .45-70 is an experience.  A bit brutal, but an experience.  I have gotten to where I just shoot light loads using Trailboss powder.  And for what I'm getting with those loads, I can achieve the same ballistics with a .45 Colt with less powder and less recoil.  I really need to get the .45-70 threaded.  A big subgun can also makes a pretty good muzzle brake.

Regarding the arm brace thing, I think they look like a cobbled-together abortion.  But I get why people do them.  I SBR'd one of my frames long before the brace thing was a thing, so I wasn't forced to chose between looks vs. tax.  I just went with the tax and installed a nice-looking stock.

I find I like rimmed cartridges in the Contender.  A .223 or similar can be difficult to extract at times.  I usually use another cartridge and hook the rims and pull the empty out that way.

There are all sorts of used barrels out there on ebay and gunbroker.  Some are great and others not so much.  You pays your money and takes your chances.  And you can have all sorts of different chambers cut if you want to go the custom route.  Lately I've been thinking something like a subsonic .32 Long would be a stupid-quiet suppressed setup.  I don't know that I would have anything I would use it for that I couldn't do with one of my other barrels, but when has that stopped us ?

Here are some ideas for you. Have fun!

Yes, you need to remove the choke to shoot solid projectiles. Not sure about buckshot.

It does have relatively shallow rifling, but I don’t know if that’s the cause of the problems. Some people speculate its the long jump from the chamber to the rifling that causes the poor accuracy with .45 Colt. I’m not so sure, as I’ve seen other firearms with a long throat that shot fine. (think .45 ACP cylinder in a .45 Colt revolver).

I suppose I could try shooting .410 slugs and see if they do any better. I don’t think I’ve ever tried those in my barrel.

I was thinking the sights may be to blame. They have a bead front sight with a U-notch rear. I find it difficult to get a consistent sight picture with this setup. But I have asked about it, and others have said they mounted an optic and still experienced poor accuracy.