That's a beautiful rifle! The coloration of the mountings is outstanding! Good for you!
I have visual problems, too. Red/green and blue/green color blind, cataracts, posterior vitreous detachment, and I'm a "glaucoma suspect." I've had some issues with targets, too. The old time "chunk" shooters used a shingle charred black, with a piece of white paper pinned to it as an aiming point, which provides a different picture than a black bullseye on light-colored paper. With that in mind, I bought some NFAA (National Field Archery Association) Hunter targets, which have a white bullseye on a black background. They look like this:
View attachment 110502
View attachment 110503
These might work better for you than a standard black-on-white bullseye, with traditional muzzleloader sights. These targets come in four sizes, but the one shown is 35cm, pasted to an 18" x 24" cardboard back for a standard target frame and stand. Most ranges provide stands and cardboard, but I bring my own. I stick the target to the cardboard back with wallpaper paste, and don't have to fool with staples or push pins. Those are spring clamps holding the backing to the furring strips... Binder clips are better, but I didn't have any handy for the photo.
Enough about targets. Let's hear some more about that new rifle!
Notchy Bob