Woohoo, I fixed it! Turns out teql wasn't actually, well, running?
modprobe sch_teql
The script now does that first. I discovered this when I tried entering the individual commands manually.
A single threaded upload test sort of appears to max out a line, but I presume that's normal. I'll probably ask on the AAISP IRC tomorrow just to see what others who have bonding usually experience. The downside to using teql is that it appears to break the EdgeRouter's ability to access the internet itself, therefore LAN IP's are now having to be routed through my Amplifi HD which I'm fine with (previously it was in bridge mode, not acting as a router).
Checking the EdgeRouter Pro 8 dashboard, on a single threaded upload speed test it appears to use one line then swap to the other. Probably just the way teql works and most probably a Firebrick is better at this.