Not an expert, but....
It sounds like the the root causes of the spots are:
1) minerals in the water
2) drops of water are being allowed to dry on the car
To keep it simple, fix those issues by drying faster and if possible using better water.
- Change your process to dry as quickly as possible after rinsing, make sure you're not leaving the car wet and spotting while doing wheels, etc. Do wheels first with a separate bucket.
- use technique called flooding the panel to dry, check YouTube for examples
- ensure quality wax/sealant is on the car
- use a quick detail spray or spray wax as a drying aid
- If all else fails use a rinseless wash like ONR, and use the Gary Dean method with distilled water instead of water frm your hose (see YouTube again for details on this...)
2005 ///M3 Estoril Blue/Gray SMG
2012 740i Black Sapphire/Black Dakota M-Sport
2010 328i Titanium Silver/Black Dakota Sport
(+1999 911 Carrera Cab, 1967 Morgan +4 SS, 2008 Nissan Armada)