Arthur, please stick with astrology. Your observation is brilliant, and relevant for anyone with a spread-out stellium in a particular sign. Because if it is not Neptune for somebody, it could be their SO's Pluto or Uranus, &c. So you are not alone.
But I think your next brilliant observation should be that you embody all of your chart. In that sense, there is nobody else out there. A lady with Neptune in Sag. could be just an all-around super person and very honest, but due to the synastry she may just appear to be deceptive to you. Are you deceptive? Probably not, but with so much Pisces in your chart you may just appear that way to an ultra-Virgo.
As universal said, you have to pick your poison. Because I don't think there are two many "vanilla" charts either individually or in synastry. Some special woman with a well-aspected Neptune in her own chart would probably be fine for you, if other signals look positive. In synastry mostly I would would look for:
1. suns making favourable aspects (egos get along)
2. moons conjunct or making favourable aspects (emotional compatibility)
3. conjunctions or favourable aspects in reverse Mars/Venus synastry. For example, her Venus conjuncts your Mars, her Mars trines your Venus.
4. Other personal planets are a great help if conjunct, sextiled or trined. The outer planets are likely to be more or less conjunct for people of the same age, anyway. Then look at relationships between them. Her Jupiter conjunct your sun, for example, is a real feel-good aspect. A well-aspected Saturn can confer feelings of responsibility to one another, rather than frustration.
You might also try looking at midpoint composite charts, as these often give a different perspective.