Excuse me for 'buttin in here and altho you didn't ask about this particular subject with your full-choked M870, don't EVER shoot steel shot in that barrel!
Over the years I've seen NUMEROUS (a dozen perhaps?) M870 and M1100 barrels blown open like a flower from shooting steel shot when the choke was too tight - usually with a "full" choke.
As for shooting slugs and buckshot, you'll simply have to invest the time and cost of testing however many numerous loads to see what your full-choked barrel will shoot well for you. There's no getting around this chore!
A Foster-style slug is usually softer lead and more forgiving (on accuracy, the barrel and choke as well as the slug itself) when fired through a tight bore, so I'd suggest focusing on those. Also, the tighter bore might also BETTER SHOOT a buckshot pattern at longer ranges (35-40 yards or more) than an open or cylinder / improved cylinder bore.
Try the standard Foster-style Remington, Winchester and Federal slugs (generally the cheapest each company makes, moderate velocity is plenty). Most hunters use "Kentucky windage" to compensate for correcting bullet impat.
Plus, #1 buckshot loads offer the maximum number of pellets in a killing pattern (ignore the #0 or #00 buck loads unless absolutely necessary). #1B is a very adequate deer-killing-size pellet.
One really great feature about your Remington M870 is the availability of accessories and extra barrels you can buy, especially with interchangeable chokes. The company below offers both Remington
slug barrels and regular field barrels for your gun too. Go to their hunting / shooting sections.