Many very helpful insights.

Two reflections from my time in a peer-review group (in addition to what you've already said):

It may be counter-productive to tell your readers what you're looking for from their feedback. E.g. if you ask something like "How is my punctuation?" you'll get tons of punctuation advice, much of which may be from people who don't know anything about punctuation. And your readers will tend to ignore issues that may be much more important. Better to let them choose what's a strength and what's a weakness than steering them in advance. Or you could establish a very systematic and rigorous protocol. But you've got to know your audience (and they have to know you) for this to work. Many readers will balk at a grid-like evaluation system.

The trick in getting feedback from readers is to gather as much feedback as possible and then look for a kind of consensus. In my group, it typically took 10-15 readers before you could see a pattern emerge. The number may vary depending on the group and the work submitted of course. But if you get 5 different people saying 5 entirely different things, you don't have enough readers for a consensus.