One thing you should look at is a music reader that EMP published in 2016. Someone recommended it to me at CASBI earlier this year, and I recently laid hands on a copy. It’s shaped note — would be great if they would also offer a round-note edition. (Maybe if they get enough requests, they would!) I’m still struggling to get my own act together for middle grades music this year, but I’m thinking I might use that book as part of my approach. There’s a section each on scale practice, timing, part singing, and song leading. It looks like the kind of content I’m interested in focusing on. (I just can’t see drilling my class on which side of the notehead the stem is on, and precisely how long the stem should be, etc. etc.) It’s a reader, not a workbook, so I’m still looking to maybe pair it with something else (hopefully something round-note to have my students comfortable with both approaches). It does give permission to copy from the book for classroom settings, which is very handy. Anyway, for what it’s worth! (And since I noticed it’s a fairly recent publication, thought I would advertise for them! https://www.milestonebooks.com/item/7-6357/ )
Also, for what it’s worth, I’ve been thinking some about collaboration. Would there be interest out there in working together with a team to get a music curriculum written that is the “perfect thing we’re all looking for” ?? And especially to address the middle-grades need?