1: The higher the EY, does not mean the better the taste is! It’s relevant if other trouble spots are ok and not, by itself, a measure of quality.
2: You want to pull, say, at least 10 - 12 shots for your best cup to establish some consistency and repeatability. However, as some one who has done a lot of data science work in the past, even 10-12 shots is too small a sample.
3: A sour cup means the extraction is probably in 15% - 17%; you may not be getting the 18%-22% or exceeding that; however, the 12-14% extractions are very low, but can be nice and sweet — not sour or bitter.
4: A good refractometer is important. The VST is the best. I believe Atago is also fine for espresso. I don’t measure my EY, although it is very useful. I don’t have one; I need a VST as I also brew a lot. It’s not a necessity, as I have other considerations at the moment.
5: I do not think there are any recommended ranges for TDS in espresso, which can taste good between 3% and 13%, as long as you can hit a range of extraction that tastes good.
6: The TDS is a matter of personal preference IMO, based on brew ratio, mouth feel, strength.