One thing on the FireWire-to-USB cables you sometimes see on Amazon — they're not protocol adapters. Some FW400 devices happen to have a connector that carries USB pins as pass-through, so those cables only work with that specific subset. A FireWire camera signaling FW400 won't cross to USB no matter what cable's in between.
Cleanest MiniDV route for the money is hunting eBay or Craigslist for a 2010-2012 MacBook Pro or Mac mini with native FW800 — $100-200, runs Sierra with iMovie HD or FCP 7 fine, real-time capture straight to ProRes or DV. Faster than dual-booting Sequoia and cheaper than a service if you have more than a handful of tapes.