But I'm concerned after watching the video below. The author claims 24 Mp is ideal and higher pixel sensors amplify grain. I'm mostly scanning Tri-X and grain is already fairly pronounced. The A7R is 35 Mp.
UGH. That's a grossly misleading partial truth. Here's what happens when you copy Tri-X. At low resolutions, the grain is mushed away completely. Some people like that! At intermediate resolutions, like the 24 MP that you are currently using, you get aliased pseudograin that is larger and more harsh than what's actually in the negative. As you continue to increase the resolution, you get closer and closer to truly resolving the actual grain. All of this, of course, assumes that you are accurately focused on the grain; if not, all bets are off. If you're being truly critical about this, a glass-sandwich negative carrier helps a lot.
In my own tests copying Tri-X, pixel-shift mode on a GFX 100S is probably the point where it's not possible to do any better in faithfully recording Tri-X grain. But for practical purposes, that's way overkill - very labor-intensive to get everything right, massive file sizes, and the difference over, say, a 60 MP straight capture is vanishingly small.
I wouldn't use an original A7R, unless it has a usable electronic shutter or EFCS mode - the mechanical shutter is shock-prone. If you're committed to using a Sony, any later A7R-series camera would be better.
BTW, the soft-edged dye clouds of chromogenic films do better with intermediate-resolution copying; the aliasing isn't so visually damaging as it is with a silver-grain film like Tri-X.
Unless you like or at least don't mind the look of the aliased pseudograin at 24 MP, in which case keep doing what you're doing and don't worry about it. But if you do worry about it, increased resolution from that point, along with careful technique, should make it better, not worse.
EDIT: Note that the above is talking about copying 35 mm negatives. If you're copying medium-format, the resolution you would need to faithfully capture the grain across the whole negative goes up - but of course, if you're enlarging less when you print, subtleties of grain rendering may be less of an issue to begin with.