Here's a thought...maybe just changing your mixing valve will help. That's a WHOLE LOT easier than changing your plumbing lines from 1/2" PEX to 3/4".
I've used Delta mixing valves in recent years; I used to use Symmons but they were becoming hard to find and very costly.
https://www.deltafaucet.com/files/l...custom-shower/Delta Flow Chart 2018-06-27.pdf
Delta makes a "high flow" shower model, part number
R10000-UNBXHF, and I've used that. It provided
so much pressure through 1/2" copper that I needed to install a small reducer at the shower head so I wasn't wasting SO MUCH water.
https://www.amazon.com/dp/B00HSWPYIG/ref=cm_sw_em_r_mt_dp_U_yAs6Db3W1K99X
This is not a political statement, just a fact: government agencies along with manufacturers are instituting all kinds of regulations designed for conservation of water. Years ago, it was low-flow toilets, and I remember people used to go to Canada to buy a new toilet that would actually flush. Washing machines today--read the reviews of some of them--don't allow you to control the amount of water in the tub; the machine calculates that, and some complaints on Whirlpool and Kenmore models indicate that some clothes never got wet! So, people have to use the "deep fill" mode. Same thing with faucets--and yes even mixing valves. Flow rates are reduced over yesteryear. You should NOT need 3/4" PEX; you can get plenty of flow through 1/2" PEX but it doesn't have the exact capacity of 1/2" copper--but with the right head, and the right mixing valve you should be OK. Where's the water saving if you have to flush a toilet twice, or have to deep-fill your washing machine each time you use it?
My current home is on well and septic. I always thought that it was the well pump and tank that were the culprits behind low flow everything in this house. While that's part of the story, if I go to the tub in this house and turn on the water full blast it's coming out at copious volume and pressure, considerably more than would be needed for a shower or sink. Thus, I reported in another thread that I went to the 'Depot and bought the highest-flow (2.2 GPM) aerators and changed them in ALL my faucets; note that they had a LOT of aerators but only two in a 2.2 gpm model.
This made a world of difference. I drilled out the reducers in the shower heads. My biggest concern now is the brand new Moen Hensley faucet I bought for the powder/laundry room upgrade. The supply lines, integral, are 3/8" OD, and the ID
is no more than 1/8". Honest. 1.2 GPM. Good enough for brushing teeth and washing hands, and that's about it.
So, take a good look at your shower's mixing valve and ensure that it is a High-Flow model; if not, I'd change that before I change all the plumbing.