Grading Grumbles

Help. I'm struggling. My grading is hurting (my students and me) more than it's helping. Something needs to change and I need to start planning for it now.

Here's where I am:

• I need to assess students individually to help them improve their musical development. As beginning and early musicians in mixed instrumentation classes of around 45, the best way I've found to do that is Google Classroom. Through Doctopus and Goobric, I grade with a rubric and try to give detailed feedback to help them progress.
• Students can take assessments as often as they choose (within the marking period) to earn a higher grade. They can also turn things in late as long as it's prior to the performance it's related to or before the end of the marking period. If they get better, the band gets better, even if it's at a slower rate.
• I've been trying to go more towards standards based grading while working in an A-E system. This has meant I've eliminated participation points (my thoughts on my Rehearsal Etiquette grading) and do not offer extra credit.
• I've gone from 12-14 assignments per marking period last year to 7-10 this year. This was in response to a few things:
• my inability to keep up with that many assignments, even if they were just credit/no credit
• a general consensus from parents and students that there was too much outside work assigned in band
• I'm very much on the "less is more" train when it comes to any and all homework. They have plenty homework in other classes and I don't think I should be adding to that just to pad grades. 
• I have weighting set up with three categories that are about 33% each (give or take depending on if there are performances that marking period). My hope was that some weighting would help balance out grades for those students whose playing isn't as strong but can demonstrate learning other ways.

Here are the issues:

• I'm constantly behind. I want kids to retry and turn things in even if they're late, but keeping on top of what's currently due for 220 students plus their late/redone entries is taking over my life. 
• With my weighting and just the small number of graded assignments, just one can have a big negative impact if a student does not do it. So even though their others may have gone fine, that one drags them down.
• Students do not take advantage of the retake system nearly as often as I'd like (even though I struggle to keep up with redone work). Many are capable of doing much better, but choose not to.
• We're still having a challenge with the idea that it's "just band" and that outside work is necessary for improvement. I don't think I'm requiring that much, but every marking period, I have students and families extremely unhappy with their band grade and even at the end of Year 9, angry emails/phone calls still turn my stomach. 
• We're in the very beginning to early stages of skill building for most of these students. Peer and self assessment can only take us so far because they're still learning a lot of the basics needed to move on. We spend a few weeks as a class on a scale or exercise before it's assigned as a test and do a lot of self assessing during that time. At some point though I have to hear them play on their own and give them feedback on how it's coming.

I want band to be enjoyable while still holding kids to high musical expectations. There is no gleeful cackling over kids getting low grades or even failing band. Mostly just a lot of second guessing as to how I can do things differently. I just don't know what else to do to keep things fair and also keep my sanity.

Does anyone have a middle school grading set up that they love? Something that keeps their students moving along, provides an accurate picture of their growth, but is fair enough that students aren't left disillusioned with band? 

Help.