I believe there may also be a second issue: multicast traffic on WiFi. If the multicast traffic is bridged over to the LAN (which by default I don't think it will be in LEDE/OpenWRT) then I think WiFi APs send multicast traffic at the lowest data rate, 1Mbps or 6Mbps for example. This means if you have anything close to 1Mbps of multicast traffic, your WiFi falls over as the multicast traffic takes all the available airtime.
I've never seen this issue myself because I haven't set up any kind of IPTV or the like, but I remember some discussing it on threads involving igmpproxy
Unless you specifically go out of your way to bridge multicast traffic onto your LAN though, I don't think LEDE will suffer from this multicast problem, which is why I suggested to just drop in a LEDE device in place of the mikrotik device and see how it goes for a few minutes (wifi should fall over basically right away if multicast is your problem). LOTS of people are using LEDE devices on 300mbps connections with success. I would recommend for a deployment like this, long term, to use an x86 solution in order to get robust handling of that kind of bandwidth, but you can do fine with an archer c7 or something like that for testing
or
for examples of how to get an x86 router solution working.
Another issue that is sometimes seen is limitations in the ISP provided equipment, particularly the number of total connections, we debugged that issue in this thread: