Lede 17 is getting a bit long in the tooth, as far as security fixes go. Sure, your ISP router's firewall is exposed to the wider internet while the lede device is not (directly), but that is still risky.
What are you doing, receiving streams or sending streams? And what speed is your link capable (please post the result of a speedtest) and what stream quality are you using?
SQM defaults to a rather fair, each flow get the same share of the available bandwidth, but sometimes with slow links and high qualiry/hifg rate streaming data, that fairness is not as nice as a less fair solution (but note without SQM there are no guarantees at all, so your high quality stream might still not work as expected). Often, for not too slow links the issue can be remedied by configuring cake for per internal IP fairness but that only works if the rate required for your streams is noticeably smaller than the total download capacity divided by the number of concurrently active internal computers.
Well for SQM to actually control the latency under load increase it needs to be in full control of the bottleneck link, so typically the WAN link. In your case that would imply that all your computers and devices are only connected to the LEDE router and only the LEDE router is connected (ideally with an ethernet cable) to the ISP router, which does not result in much load sharing between the devices unfortunatelly.
With appropriate per internal IP fairness that might still work, but your TP-LINK MR3420 v2.2 really is not powerful anough for your aggregate ~150 Mbps. So you really should retire that device or if not an option reduce the shaper considerably to gain some CPU cycles back.