Should I replace my trunk-and-branch setup with a PEX manifold?

Personally I would just live with the trunk and branch. It will be very expensive to replumb the house.

If the manifold cost is not exorbitant, I would definitely go with your plan. I can hardly believe that anyone would use a 1/2" line for a trunk. I'd be tempted to use a 1" trunk if you have the crimp tool or can borrow one--especially to feed the manifolds (and your incoming line is 1" or larger).

If you increase the diameter of the branch you also increase the time it takes for you to get hot water. In my climate I also like to get ground temperature water so having large branch lines increases the amount of water I have to dump before I get rid of the water in the pipe from the supply entrance to my faucet. This also wastes heat as the branch lines will have hot/cold water and heat or cool the wall spaces - eventually heating or cooling your house.

I like the manifold arrangement but one quick thing to try would be to just plumb the washing machine into the end of the branch line. I this this would allow the other fixtures to have pressure preference over it.

The other thing I'd do is eliminate the crazy 90s if you are redoing a section. The plumbing might look neat but you loose significant flow when you make sharp turns. Really the best way is spaghetti with gradual bends.

PEX is a dimensionally equivalent product. That means the OD outside diameter is equivalent to the OD of copper. The ID inside diameter is less than the ID of copper. The PEX fittings fit inside the PEX tubing acting as flow restriction and thus volume of water. Do not think that the rules of thumb for copper are equivalent for PEX. Normally a 1/2" copper line will feed three shower heads or three faucets with a static line pressure of about 50 psi. You state problems with water pressure due to plumbing. Some people experience extreme variations in shower hot water temperature and equate it to a water pressure problem. It is normal to have hot water temperature variations due to more than one person living in a home. There are several work arounds for this problem. Install a hot water recirculating system and/ or install anti scalding shower valves. Both in good working order should reduce hot water temperature variations.