Does your router have a Master Browser setting? If so, turn that on and then reboot all the machines, and see what happens. It might be worth finding out which machine is currently operating as the MB as well...
ASUS GT-AX6000 v2
V2? I thought there was a discussion elsewhere that determined there was only one model GT-AX6000? Or did the original always have that model notation?
I don't know whether or not it's a confirmed fact or not, but I've read in this forum that iPhones are particularly sticky to an access point, not wanting to let go and switch to a stronger signal quickly.
Are you using the Speedtest website or the Speedtest app available from the Microsoft Store? It's recommended to use the desktop app as a browser test can be encumbered by settings or extensions being used by the browser when using the website version of the test...