Sky Watcher Pro Series Mak

Jim, aperature rules. With the same mirror size, longer focal length
gives you a small secondary. So, while 10" f8 newt would potentially
surpass your 10" f4.8, a 6" f12 has far too much a disadvantage in
size. Also, Mak's have a relatively large central obstruction.

To top what you already have (which is a very good scope), you could
get a larger newt, a same-size but longer focal length newt, a larger
SCT, or an apo refractor of the same or possibly slightly smaller
diameter. Those are all massively expensive options. I'd be more
inclined to invest in better eyepieces for the scope you have.

Greg