Well, i finally got it.
Turns out my primary mistake was inheriting mender-full in my image rather than local.conf.
That lead to MENDER_FEATURES being empty for anything other than my image recipe, and thus everything went haywire.
A second problem, was mender setting the wrong root device. To fix that, i had to move MENDER_STORAGE_DEVICE into local.conf as well. All other mender configurations seem to apply just fine from the image recipe, including MENDER_STORAGE_TOTAL_SIZE_MB, just these two were causing issues.
I was trying to structure my code logically, and isolate sections as much as possible to avoid the bloat to local.conf, but BitBake has other ideas it seems.
Thanks for your time @annalenamarx and @TheYoctoJester 