The difference from that 1.3.0 (of Jan 2019) to the 1.4.0 in 19.07.7 is
https://github.com/tohojo/sqm-scripts/compare/4d6ec3349...ab763cba8
@moeller0 can better evaluate if there is anything crucial, but it contains quite a lot of changes. you might be better off with the newer sqm.
The network config looks rather normal, I think.
Understood, is there any way to push that package (update) without upgrading my current firmware?
Assuming the issue is with the actual sqm-scripts and not some of the olther core components like tc/ip/kernel modules, the followong site has instructions how to upgrade to the latests sqm-scripts:
https://github.com/tohojo/sqm-scripts
And yes, it would be nice to continue debugging this with the most recent version of SQM 
Hey guys, i know I'm late to the party but this issue has been giving me a massive headache.
I've just fixed by removing WAN6 interface, hope this helps someone else!
Cheers
Thanks for letting us know! Will definitely try this. What I do is restart sqm in system-startup in luci config and that seems to work for a while at least.