SQM on R8000 Nighthawk bad download speed?

Ok, so your tests don't indicate significant network issues. based on your wifi analyzer screenshots your 5GHz APs seem fine. I would move the one that's currently on channel 6 to channel 11 so there's no overlap with the "dlink-CD87" access point.

What kind of device is the

This maybe just doesn't have the CPU power to decode better streaming?

Its a samsung xcover pro mobile phone, doesnt matter if its from Dell laptop or hp6570b pro, Samsung tablet a7 or any other wireless device we have, they all share this limit.

I doubt all devices are this bad, my friends hacked nintendo switch with android does 80mbps with moonlight at his home and the wifi chip on nintendo isnt something to write home about or so ive heard.

You're suspecting device issues?

Edit: thanks for the tip regarding 2.4ghz band but ill soon be adding my third AP and that will populate channel 11, meaning I will have 1,6 and 11 populated.

Try just for a moment turning off 2.4GHz entirely, see how things behave. Your 5GHz indicates essentially no interference, if the real issue is 2.4GHz interference let's discover that by forcing everything through 5GHz.

Im not streaming through 2.4ghz wifi thought, I keep 2.4 and 5 on different ssids.
They interfere?

No if they are different said then you know what you are using. They don't interfere at least radio wise. They will both use CPU so could interfere that way if many devices use both channels. Sounds unlikely to be the issue

All tests I've done is as isolated as possible, if that's the right term. Only 1 device at 5Ghz band at the time - I'm not really sure whats going on either since streaming is done locally - in my head I should be able to bump up the streaming bitrate by alot.
Such a shame if its down to the wifi clients, I mean.. they are no flagships but it shouldnt stutter like it do and certainly not occassionally as it does on just 30mpbs - the devices were quite expensive when they was new and they aren't that old. Samsung premium price? I dont know..

I tested with 2.4ghz wifi band shut off and there were no difference, Im starting to think I might aswell sell this R8000 router on the market - apart from the signal strenght the R8000 puts out I see no difference between this router and the unifi setup I have, I have a spare UAP AC PRO not being used so if I plug that in I would get the same coverage as the R8000 bring and it would certainly only be for the looks since the R8000 does a better job than two AP pros on the opposite of the house do.. so strange.. maybe I need to pay someone to have a look at all this, im not happy.

I appreciate your help! your awesome, thanks!

Proper tuning of the radios goes a long way. This means ensuring that your APs are using non-overlapping channels and that the power of each is set to the minimum power that provides coverage of your space with minimal overlap of the two (or more) AP coverage zones. A wifi site scan will help you start mapping out what channels work best in your space (and at each AP), but most of the time there must be some compromises unless you live in a very RF quiet area.

This video describes the process of placing and tuning your APs. The concepts apply to all wifi APs, but this particular video does it with Unifi (side note: the video is a year old, so the GUI may be a bit out of date if you are keeping the Unifi Network Application (controller) updated). You can use this to try to improve your Unifi setup, but it will also be equally valid if you go with any other system.