An extra, low rear sight mounted in the central rib might be just great, John. How the gun puts the slugs into a group, is the important thing. Mounting a red dot sight might even be better, but more difficult.
Practicing with a fox target is a great idea. It will tell you at what range you can be lethal on them with buckshot.
Back in Ontario, BB was the preferred shot for fox. I actually preferred the .264" size, from my 6.5 magnum. LOL The .22 Hornet did just fine on them as well as the .22-250. THOSE were the days of fox bounties.
The larger sizes in buckshot, over about #4 buck, might be too large in pattern, with open & loose patterns but testing is the only way to show how your gun handles them.

The typical FOSTER type slugs are still a VERY good projectile.
As far as cast slugs go, the sabot slug from Lyman is supposed to be exceptional. I have the mould, but have tested them only minimally & in a rifle barrel. The slug is about .690", 525gr. in pure lead and fits inside a 12 bore wad, typically the Red WWAA12 "field" wad which is a 1 1/4oz. shot wad in 12 bore.
Now, how that slug would handle in choked guns, I do not know, however due to it's size, hollow base and narrow driving bands, fore and aft, I assume it's safe for any choke. 12 bore typically is .690" in full choke (that's 40 points (thou) of choke). Modified is about .710" to .715".
That's about the only slug mould I am aware of in 12 bore.
There are ALWAYS round ball loads. I cast up and supplied my local friends with .682" pure lead balls (482gr.) that they loaded with 1 1/8oz. shot load data, but with a card wad and fiber on the powder, then a cloth patched round ball, then normal folded crimp. That was for moose, when they were duck hunting, as happened now and then.
There are more powerful loads for round balls (they used 690") in some of the Lyman shotshell loading manuals that I have.
Lee makes a .690" mould and they & Tanner moulds are inexpensive compared to Lyman or other makes..