New plumbing - Lose Water Pressure in Outdoor Hose
If you have 2 seconds of good flow followed by low flow there is a restriction upstream.  The longer the burst the further up stream the restriction is.
The leak at the ball valve stem, You may have melted the oring or nylon seat that seals the shaft when soldering. The heat deforming the seal causing the leak.
I have had luck removing the nut and handle then the nut on the shaft. Orings are easy and cheap the ones that have nylon sleeves can be a bit tougher to find.
In some cases I have purchased another valve same size and brand and pulled the seal out to repair but most have been orings.