They make a fitting called a "drop ear 90" which is a pex connection on one side and a 1/2" ips (iron pipe size) female on the other. The "drop ear" indicates that there are flanges through which you can screw it down. Excellent piece. Readily available! I've used them on my current remodel of baths, laundry and kitchen and they are easy and secure.

I debated about getting one of those fittings, but I wouldn't be able to screw it to anything. Unless I built a little ledge out from my floor joist.

What if I went with a 1/2" pex to 1/2" female ips, screwed in a short 1/2" brass nipple, then went to a brass 90, then went to a chrome nipple, then to my supply valve?

I would secure or strap the brass nipple to the floor joist, kind of like they did in the picture above.

Too complicated?

You are looking at it. You get to decide. I can see the picture but understand your concerns. Either will work. The real deal to me is that you have the lower piping secured so you can take that chrome trim nipple and with a strap wrench be able to tighten it sufficiently to prevent leaks and not have the torque twist the pipe.