The real weakness of any MF scanner is in the film holders. I spent a while chasing better results on my V700, including buying an aftermarket holder that was over $100(from BetterScanning) with a glass insert. It was better than the stock Epson scanners, but even after I spent hours dialing in the focus the results were still less than stellar. My best results came from wet mounting on the Epson glass plate, although this has a steep learning curve(most of my "good" scans from doing this still have bubbles), is a royal pain, and can also get pricey in terms of the initial investment and the ongoing cost for overlays and mounting fluid. Of course, any mounted slides will have to be unmounted.
I now do all my MF scanning on a Nikon Coolscan 8000. This will cause the same issues you had with your Canon in that Nikon hasn't supported this(and the nearly identical 9000) in years. The Nikon software is better but in my experience gives better results than Vuescan, especially if you are using IR cleaning. You need a Firewire port(you can still get a TB-FW dongle, so that's not a huge issue) but you are stuck with Snow Leopard or earlier if you want to use the Nikon software since it is PPC native. Before someone says it, virtualization doesn't/can't doesn't work with these scanners as FW pass-through has never worked on Macs.
The Nikon holders are also poor. I modified mine to hold a sheet of anti-Newton glass and can consistently get across the frame sharpness. There are two different Nikon glass holders, but these bring serious money on Ebay-nearly as much as you'll pay for the scanner. Also, the FW control boards are a known weakness on these scanners and you have to always be mindful of how you're connecting and disconnecting them.
If you want to throw bunches of money at it, get an Imacon, although you'll be stuck with unmounting all of your stuff and you have the same wet mounting headaches.
That's basically your options. As much as I love getting big 6x6 or 6x7 chromes back from the lab, with Cibachrome now dead a lot of my MF shooting(I do still actively do it with a Hasselblad and a Mamiya RB67) is in B&W and we printed in the darkroom.
