I've decided to make a post here rather than the issue tracker because I'm new to this and I'm not sure I'm right.
Anyways, here's what's been going on. I have a NETGEAR Nighthawk AC2400, which according to this commit, is equivalent to the NETGEAR R6700 V2. So I went and tried to flash the factory image, but all I got was a bootloop. Using nmrpflash, I recovered my router and found that all my settings had remained unchanged (not sure if this is normal). Now, using telnet on the stock firmware, I took a look at my routers partition table and found that the kernel partition is a different size than what is listed in mt7621_netgear_sercomm_bzv.dtsi.
So I see two different possibilities. The first is that not all of these routers have the same partition sizes for whatever reason, which seems plausible after reading this thread. The other possibility is that my router is not actually equivalent to the R6700 V2.
I haven't actually confirmed that the partition size is what caused my router to be unable to boot OpenWRT, but it seems highly likely. If it turns out to be the case that my router is equivalent to the R6700 V2, I've been looking at these two patches which might fix the issue (1 and 2).
If anyone more knowledgeable could chime in that would be great!
edit: forgot to post the picture of the parition table.