The job ended up taking a bit longer than anticipated, but after much sweat, some cursing, and only a minor injury, I ran the new power circuits as planned.
The root of the problem I ran into was that it was not as simple as I thought to pass the wiring up from the main and into the attic. At first it looked like there was a clear shot and the wiring would be able to be fed up through an access door in the garage I'd installed for a previous project into the attic. When it became clear that it wasn't going to be as simple as grabbing the wire being fed up from above, I knew I was going to need to build a platform in the attic to work on, as that part of the attic is where the headroom is shrinking to virtually nothing, and it was going to be hard enough to do without falling through the ceiling when I thought that all I was going to need to do was grab the wires being fed into the attic.
Step one then became to make some room to work. That wasn't a very complicated or particular difficult process, but it was pretty time consuming. I had a couple sheets of plywood sitting around that I cut down into strips to screw down in the attic to give me more room to move and work so I could get a better look at was going on.
Once I could comfortably / safely get above the main to see why I wasn't able to feed the wiring through I was able to see the problem was just that there was some of the expanding spray insulation that was filling the gap I had been hoping to use. It was a simple enough solution of pushing the fish tape through the foam and then pulling the wiring back up and through.
After that the job was about as simple as I'd first expected. I hit a slight snag when I found I wasn't going to be able to get the wiring back out of the attic at the spot I was hoping, which meant that using rigid conduit like I'd been planning was going to be problematic, but got around that by using some flexible conduit I had instead.
As a bonus I didn't die working in the main. I've done a lot of electrical work over the years, and either through that or just incidentally, I've taken a lot of shocks. But I still get very nervous every time I open up the main. I'd say justifiably. That amount of power is no joke.
As a further bonus, I now also have a lot of that legendary attic storage space that my wife has always been asking for.