Water flow/pressure drop

When I was installing a tankless water heater in a new location – I decided to replace my 1/2 copper with 3/4 pex as part of the project. However, after turning on the water back on if more than one faucet open- regardless of hot or cold. It isn’t so bad shower still flows but 3 would be annoying. Since it happens to all cold as well as hot water I am not blaming the water heater – it can handle up to 9GPM anyway(or 5.4GPM @ 60F rise).

So I think it might be one or more of three things:

1. The water meter might narrow done to 1/2” – I noticed this after checking if the valve was opened all the way. It goes from the city 1” to 3/4 to the meter and back 3/4”. . Is that 1/2 (what looks like a union) going into and out of the meter? Even though there is 3/4 copper afterwards. Would that render the upsizing the pipe useless? Or cause the drop in water flow rate?

2. Too many elbows in the pex – I just finished repiping the gas line and I guess I had 90 degree elbows in my head. The main cold line to the water heater has 6 elbows. Would that have an impact on water flow rate? Would a mostly solid line with bend supports make a noticeable difference?

3. 3/4 pex inner diameter almost is the same as 1/2 copper. And when using couplings and those plastic elbows seems to even make the inner diameter smaller. Are there pex fittings that do not affect the inner diameter? Maybe the plastic shark bites?

4. I did not set up up a manifold with home runs as I wasn’t planning on repiping all the copper. Should I set up a manifold with home runs to each outlet or at least close to the outlet. If so, the upstairs bathroom would be difficult.

   Please let me know which if any of things above should be corrected. And if there is anything else I can do to increase the water flow rate when multiple outlets are opened.