I was quite amused to see this posted as we just built nearly the exact same thing about 6 months ago (minus the NVMe drives which weren't available). We are using ZFSonLinux on top of DRBD instead of mdmadm, which has been surprisingly successful.

The biggest frustration that we met was using LACP in trying to get a better data rate out of DRBD. Regardless of the LACP mode, we were not able to get DRBD to scale to full bandwidth (20gbps) and had to settle for using it for redundancy only.

I imagine with your setup (read: much faster peak write capability), you might actually be able to bottleneck DRBD pretty well over a single 10GBE pipe if you were writing at peak capacity. I'll be interested to see if you happen upon a work-around.

Load balancing as you've suggested is an intriguing compromise. Have you tried doing a two-way heartbeat setup where each both boxes have their own active IP and can fail over to each other?