R8000P - Random bandwidth drop on devices connected over WiFi

I have a Netgear Nighthawk R8000P running V1.4.1.68_1.3.28 . Recently my WiFi connected devices (both on 2.4G and 5G) are randomly suffering from loss of bandwidth. The router is currently in "Router" mode and has a PPPoE internet connection at 300Mb/s (checked directly on ethernet as well as on the on-board speed-test.
There are many devices on 5G WiFi: 1 iPad, 2 Laptops, 3 Smartphones, 1 Chromecast audio and on 2.5G WiFi there's a Smart TV, 1 Chromecast 2 and one Sonoff 4ch.

Occassionally some of these devices (at times all of them) suffer serious bandwidth drops where they report speeds as low as 750Kb/s. Sometimes even 60Kb/s, while the other devices are still reporting 250-300 Mb/s. Disconnecting and reconnecting to the WiFi restores the bandwidth to 100%. Ethernet never suffers from this issue.

I've confirmed that there are no other WiFi signals on my 2.4G or 5G channels (to ensure that the issue isn't because of interference).
QoS is disabled, traffic monitor is disabled. I've even factory reset the router and configured it from scratch and still am facing the issue.
There are frequent disconnects between the devices and the chromecast and also random WiFi drops on the iPad saying "This network has no internet access" (Which is funny because a speedtest on a smartphone next to it about 2 seconds later gives me 300Mb/s).

I first suspected the Sonoff so I left it powered off for a day or 2 and the issue still exists. Any suggestions would be helpful.