I found that video:
I thought it was more fun to watch than endless side-hilling through the trees. That's partly because you're always guessing where that sled is going to peter out, but it did a lot more than I'd expect with such a short track (137, I think). My old sled is very similar: a little better ergonomics and suspension, but still a wide sled designed around sit-down riding. Also, my sled started as a 136, but now is a 144. That's still not enough for deep snow, other than maybe motoring around on fairly level ground, but it makes a more of a difference than you might think.
Muskoka makes sidehilling it look easy - even with a broken ski! - but you can see where even the best rider can't make up the deficits in some areas, like going uphill on edge. I'd say the short track and wide body constantly paneling out are what's holding it back. I'll bet that wore him out pretty quickly compared to a newer sled too. You can band-aid that kind of sled with handlebar risers, narrowed front end, suspension tweaks, etc., but at best, you're only going to make up a third of the ground to a new sled. I'd put a sled like an M8 in a transitional generation, along with the Polaris iQ and Doo XP. They weren't that big a step up in raw capability, but they're worlds better to ride on edge. You could make up more ground modding them, but it makes no economic sense unless you're buying used parts and are pretty handy.
So basically, I think it boils down to "buy the best sled you can afford," but I'd add that while sledding has a higher price of entry than a lot of passtimes, it's something that'll eat up every penny the average person is willing to throw at it. And it's easy to burn through a big chunk of savings or rack up debt if you get caught up in "gotta have it" fever. What I love about that Muskoka video is it shows you can have fun on "old junk." It's all a matter of expectations. Now, I'd question your life choices if you insist on running around on a sled like that Summit or my old 600 when you could afford something like an M8, or I'd say better yet, a Pro, Doo XM, or Ascender. Dare I say, there's more to life than sledding, but what I've done is to build my budget and upgrade during the off-season. When you're still dreaming of powder in July, working an extra shift or whatever side-hustle is well worth the reward. Anyway, lots of good advice here, and I think you've got a good plan for moving forward.