Update.
First, I was able to borrow a power washer from a friend. Score!
I had great success in cleaning up the garbage bin, recycling bin, part of the fence, the mildew on the house siding, etc. Would do the outdoor furniture, but stuff that couldn't get sprayed was in the way.
I had great success in cleaning the INSIDE of the garage. It is no longer massively filthy with an inch thick layer of dirt over much of the concrete. I will not be turning the electricity back on for a few weeks, just to be safe, since I sprayed everything. Also cleared out a bunch of junk.
However, the paint did not come off the exterior. I am hopeful that it will be loosened because the wood is so wet, so I could scrape it off without a massive struggle. I'll check it later with a scraper. Maybe have had different results with a stronger machine or different sprayer, but realistically, I'd rather not spend the money to find out.
I have no idea what this type of siding is called, but it's very common on wood garages I've seen. It's not flat, and it's got 2 "slats" per piece of wood. There's a curve on the edge of each piece. Sanding is not easy as a result. Been down that route - this is the same style as my parents garage, and we tried sanding their garage
While I don't like the idea of it, if the top layer of paint doesn't loosen, I may just live with it until it does. Problem is, the garage is wood, and it will deteriorate without a protective paint layer.
If I was willing to invest the money, I'd tear down the garage and rebuild it, because it's got issues. However, I am not willing to spend the money on that project right now. Maybe someone will be kind enough to build a catapult and fling some boulders on my garage. I'm good with insurance paying to build a new one.