Just gonna throw this out there; a 12v suffers from every restriction and bend. The proportioning valves and the 90, the hose barbs down to 1/2" on the supply side, etc.
Feeding with a smaller ID than your output isn’t doing you any favors. My 7gpm 12v has a 3/4" hose feeding it: no 90s, no proportion valves, and 200’ of 1/2" hose into a pvc j-rod setup. I can just about shoot an arc over a 2-story
If your water line don’t have a metering valve on it, you won’t get accurate % of mix coming out. That’s if you’re using a ball valve for water. The metered valves are more restricted than the ball valve. You won’t be able to run ball valve wide open, will need to be tweaked a bit.
There’s some good info here and I agree that you really only need 1/2" for that pump. One question that I don’t think has been asked yet is did you pull all the hose off the reel for your test? You need all the hose stretched out so it can flow it’s best.
Get rid of the sutner and twist, those are for soft washing with a pressure washer. Use ONLY a ball valve (sch 80 preferred) reduced down to 1/4" and get a jrod with xx60 or xx50 (xx = 0, 5, 25, of whatever you prefer). Once you find the tips you like, DONT change them otherwise you will always doubt yourself. Too many times have I had things perfect later to find myself watching a YouTube video and then doing a complete remodel of my once perfect set up. Find what works best for you and go make money! PS - Stay with 5/8" hose, anytime you can have more flow at the spray tip you will do Great. Just like pump supply lines, Hose diameter should be double the inlet which prevents cavatation and starvation. You want your supply to be heavily ready at all times.