Every gun is different so you'll likely have to shoot various bullet/sabots and powders, maybe even primers, to find the gun's sweet spot. Either of the bullets you mention are shooters as well as great hunting bullets inside of about 200 yards. The sabots I like as well as all of my guns are the Harvesters but MMP makes good a product too. I think most people begin with Harvesters. The powder will steer you big time. If you're going to use BH209 and are using a volume measure, I'd start at 90 grains and increase by five grains every five shots and cleaning the plug's flash channel at each increase. Lots of the 50 cals using those bullets like 100 to 110 grains by volume. Personally I'd stay away from pellets of any kind. T7 granular will give results very close to the BH209 and is way easier to find right now so maybe begin with that powder. Sabots are relatively cheap so perhaps get a regular sabot and crush ribs for each bullet caliber to do your load work up. Winchester, CCI and Federal are all good primers to begin with. I use Winchester shotshell reloading primers in all of my guns as well as CCI when I'm playing at the range.
Others will chime in I'm sure.
Nice looking gun by the way.