I was thinking I needed a separate container so I could use my little mixer, but I see it comes with a stick attachment that would fit in my bottles, so maybe I won't have to bother even with my Pyrex measuring cup. Again, good. The less liquid I have to transfer from one receptacle to another, the better, since pouring is fraught with peril in this household.
I use a little
Badger paint mixer, which is similar (in principle at least

) to the Norpro. It doesn't fit the narrow neck of my unicorn bottles; so I use beakers to mix in.
Using a mixer; I have learned a few things, that seem to work for me. Leave enough room for temporary liquid expansion (while mixing). That means, for beakers... I try to use a beaker that is 33-50% larger than my desired batch size. I once tried mixing a 10mL batch in a 10mL beaker, and half of it was left splattered on the counter.

Too big of a container is problematic too. Mixing a 10mL batch in a 100mL beaker, just leaves most of the liquid splashed, and clinging to the sides of the beaker.
Out of the 10, 25, 50, 150, and 250mL beakers I have purchased; I find I use the 25 and 50mL most often. The 10mL seems to only be suitable for 5mL single flavor testers. The 25mL works well for 10-15mL batches. And, the 50mL is perfect for 30mL batches. I usually don't mix larger. But, for the rare 60mL batch, the 150mL does okay (it is a bit big... but functional).
The one exception to this; is my unflavored ADV. That, I mix up 250mL at a time. For batches that big, the Badger is just too small. I mixed that one in a repurposed (washed) large (1qt.?) yogurt cup, and use the kitchen immersion blender.

Mine happen to all be the borosilicate glass; but plastic is just fine... and can have advantages too. Glass is heavier, and depending on your scale's maximum capacity, can limit (to a greater extent) batch size. Plastic, being lighter, leaves more total combined (liquid+container) weight to the liquid. Plastic may be mass produced, and the graduations less precise; but it doesn't matter as the scale is doing the measuring.

Plastic is also cheaper, and much harder to break.
As for having another container to wash... it has never been a problem for me. A quick hot tap-water rinse, dry with a paper towel, and I am ready to mix my next batch.
Transferring
can be a bit challenging. My hands are not the steadiest (another reason I like the wider mouths of the breakers vs. mixing directly into bottles). But, I have found that a small funnel alleviates that hurdle. Washing is the same hot water rinse.
This is by no means the "only" way, "best" way, or the way I am saying you should mix. It is just another way, that happens to work well for me. Successful DIY is all about finding a process that works best for you.
