if you really want a deep dive (cuz your post suggests that), fire up wireshark on a speed test PC and capture the traffic while you test. look for out-of-order, missing and corrupted packets. you will also get awesome stats on the traffic - wireshark is your best friend. be warned, this is the entrance to the rabbit hole.

edit: because at some point you are going to want to slide wireshark between the cable modem and your router - for general troubleshooting (and funsies!) then things get interesting as you figure out how to do that properly.

test it early morning when your neighbor peeps are sleeping (or bulk traffic torrenting - QoS usually knocks them back down pretty quickly). at the least you are looking for rock solid ping times. if pings are wild your link or the community bandwidth is possibility saturated.

is something already pushing 300Mbps worth of traffic across the router? speed tests that are good at power cycle, but quickly deteriorate after, can sometimes indicate that you have unaccounted for traffic crossing a bandwidth limited i/f (your ISPs service).

check your router stats for the missing traffic.

edit: also, almost all ISPs have a “burst bucket” for quick but intensive bursts of traffic. you get super speed for a few seconds while your bucket fills up. once full/overcommitted, your ISP starts rate limiting your service again. that may be why you get nice initial speeds, but they drop off quickly. does your ISP give burst speeds and sustained speeds in its terms of service contract on your kids?