Glad to hear this, but if I understood right: testing is not done by "cooking" bin's and flashing them in emergency/recovery mode (which accepts "any" firmware)? There shouldn't be a need for tftp.
Look, dir-859 works flashing openwrt in emergency mode straight away: D-Link DIR-859 A3 AC1750 works with DIR-869 A1 image
Essentially what else would we need?
Sorry if I appear too simple, but: take openwrt from dir-859 (edit: 869 also similar, all revisions?), put qca9888 driver and the hw addresses found in bootlog. And done. It should just work, isn't it?