This is quite asymmetric, your upload will be (up to) ~50% loaded when you saturate the downlink side from the ACK packets alone. Are speedtests really telling you that your upload is that low?
Well, that really depends on what is transmitted over the trie bottleneck link, so the wireguard overhead should be added, not sure about ethernet (which amounts to either 38 bytes or 42 bytes with a VLAN tag), if in doubt, pick the larger number...
Okay, the question is why, is the router's CPU overloaded? If yes I would expect relative high numbers reported in the av_delay and pk_delay comumns of cake's output from tc -s qdisc.
Not on fixed networks, no, but please post a link to one of your speedtest reports (maybe also try dslreports' speedtest?)
Sorry, interested in that myself.