Read the spec sheets of the devices you want to feed. Add up their nominal outputs. Showerheads only work well when you can provide more water than they can use. Figure out what the maximum fixtures you want to have running at the same time. Jets tend to be around 1gpm, the rainhead and shower head are typically 2.5gpm each, so the rough estimate is 9gpm, yours may be different. No way will you get that from 1/2" pex, and none of the 1/2" shower valves will flow that much, either. I'm not saying you won't get any flow, but because with the volume they really want won't be satisfied, there won't really be the design restriction and the outlet won't speed up. When you put your thumb over the end of a hose, the spray may go further, but the volume is likely the same. The restriction causes the velocity to go up. Then, have someone turn the valve down, and it doesn't go as far. The pressure supplied is identical, but you've restricted the volume. You can verify that yourself if you buy a pressure gauge, screw it onto a hose bib, slightly crack the valve open and read the pressure...it will stay the same whether it is barely cracked open or opened fully after a short delay while it fills up the sensing device.
Throttling a line does NOT increase the pressure in the way you're thinking. WHat happens in a showerhead is that the restrictions in the jets, by the Bernoulli principle, cause the fluid (whether gas or liquid) to accelerate, but the actual volume may be less, depending on where that restriction is.
Now, if you ran that 1" pex line to a 3/4" valve in the shower, it would work fine.
FWIW, whether you have a soda straw or a fire hose, the pressure is the same...it's just the volume available will be drastically different. Because of the radius-squared factor in the cross-section of the piping, a 1/2" line can carry half of what a 3/4" line. 1" pex is slightly larger than 3/4" copper, so would not present a restriction for volume to a 3/4" valve.