I actually like to use both composite and synastry charts. I look at them differently though and don't use every little detail as I am looking for the strongest energies I can find as quickly as I can. Now when I would begin falling in love I would do all of those details but as time has passed I like looking at a chart and what "pops" is what I focus on.
I like composite charts because there is only one chart to interpret so right there is simplicity. And I have found them to be pertinent for a relationship. When looking at a composite chart I can see conjunctions easily and if there are tough planets involved that sends up a red flag immediately. Say there is a Venus/Pluto conjunction. Right then I know that there might be some control issues regarding the love energy. A Mars/Pluto conjunction or opposition also sends a red flag of possible domestic violence problems. A Sun/Neptune square could denote some deceit in the duo. The list in endless but each one of those examples give me the energy that is going to be most trouble some. Moon/Venus conjunction is really nice in a composite giving a loving emotional vibe to the couple.
But occasionally I have seen a composite chart of a 20 + year relationship and was quite shocked at the multiple T-Squares that I would ordinarily say "No way" but instead (since I knew them well) I found that they had many red flag issues early in the relationship but they also had some good aspects that helped them overcome these problems through compromise and persistence. So when looking at a chart it can seem impossible but through free will we can overcome tough aspects because all hard aspects are nothing but Life Lessons. And we can either have the desire to surmount those issues or walk away from the relationship. Which is the easiest path? To walk. But there are couples in this world that desire to stay together so they had worked through most of these Life Lessons and I had to give them an A+. We do have free will after all.
And another thing I have noticed is that when there are issues in the composite chart that those same issues are prominent in the synastry chart. So they both yield similar results, Well that has been my experience anyway. A hard Neptune aspect in the composite is often in the synastry chart too for example.
In Synastry I tend to look for (again) the conjunctions as they most often are the most apparent aspects and are the most powerful aspect. The nice thing about synastry is that you have more information regarding "who" is doing what to "who." When ones Sun conjuncts the others Venus this is a strong indicator of one that heavily "orbits" (Sun) the Venus of the other causing the Venus person to KNOW that the Sun person is totally interested in having them as their mate. When ones Saturn conjuncts ones Moon there is probably going to be emotional issues and a feeling of not being "understood" or even emotionally "cared for" by the Saturn person. The Moon person would feel that their feelings aren't given any attention as Saturn isn't the romantic of the zodiac for sure.
So you are correct in how you view synastry and composite charts and I do like to see both. For they contain similar but yet different ways of interpreting the relationship. And they both work. But for speed I do like the composite as it is obvious what the issues will be and the energy of the relationship itself. Gotta take the good with the bad.
But one thing I am beginning to have issues with are all of the asteroids and points and even "empty" planetary positions where there isn't even a planet. The major asteroids are one thing but when the zillions of asteroids are added to a chart I just feel that it is "too much." I would rather work with a traditional chart at first anyway to get a foundation to build the rest of the chart "on." And then adding Chiron and possibly the other major asteroids at most. But I do believe the asteroids are important in this 21st century astrology but sometimes I just want to shout "Enough!" When there are tons of details in the chart it feels like I can't see the forest for the trees. Frustrating for me anyway.
Hope this is somewhat helpful and good luck and bless you as always. I wish you well.