Indeed, there could be a combination of factors at play.
However, your low volume of yeast made points towards it being a growth problem (not enbough oxygen, nutrients, viability or count)
For a plethora of reasons, milling fresh will become inevitable at some stage. It depends on how far you want to take it and your determination. German breweries are superb masters of chemical engineering. The idea with the pot and the wooden paddle yields a result, but if you want something as well made as the German beers you like, then approaching their setup will become inevitable of sorts (there is some variation though).
Milling in itself is quite interesting since you can condition the malt and reduce LOX if the water is at 80C.
I'd consider a pH-, DO-, refracto- and calibrated thermometer alongside a malt mill and the ability to perform temperature controlled fermentation as the minimum equipment of any serious brewer
Thus said, if you just pitched yeast from vials then it's a crap shoot. Yeast loses its viability over time, so without an appropriate propagation step (or steps) it's a crapshoot.
I often heard people here say that one should never brew before the yeast is ready. It's great advice and I certainly heed to it.
I'd try that first before investing in more equipment. After all, you can easily test this without investing into more equipments.
You can also try a mash pH 5.4 and a 40 min rest at 64C.