Given that your original problem was related to WAN upload performance, why did your investigation lead you to Ethernet flow-control? An ISP connection generally deals in packets at Layer 3 ("network", eg IP) of the OSI model, whereas Ethernet is a Layer 2 ("data-link") layer technology.

If there is a bottleneck at your WAN modem, then that will cause congestion at layer 3, but Ethernet flow-control can only deal with congestion that exists at layer 2. What has likely happened is that you have configured your gateway so that congestion at layer 3 is mirrored onto your layer 2 LAN. And if flow-control is enabled, then that would result in back-pressure propagating back to your VMs. Your VMs will then slow down their layer 2 rate, which conveniently forces the layer 3 traffic to also slow down.

This is an incredibly round-about and inefficient way to do traffic shaping. You should not configure a network so that L3 and L2 issues bleed into each other. A major consequence of using flow-control in this way is that it reduces the capacity of your LAN, even for traffic that isn't going out to the WAN.

The customary approach for keeping L2 and L3 separate is to perform traffic shaping solely at the threshold where your LAN meets the bottleneck. This would be OpenWRT, since after OpenWRT would be the WAN (50 Mbps upload). OpenWRT would be configured with some sort of QoS feature so that certain L3 packets are selectively dropped.

You cannot do effective L3 traffic shaping without dropping packets. In fact, all competent L3 protocols expect dropped packets in order to slow down their data rate: SCTP and TCP have their own exponential congestion control mechanism, UDP simply accepts that some packets won't make it through, and QUIC has its own mechanism as well. Simply put, all L3 protocols only understand one signal that tells them to slow down, and it is to drop a few packets. They will adjust accordingly, finding the stable equilibrium where traffic flows at the very cusp of congestion.

The Main reason for this problem seems to be the down-stepping of 10Gbit traffic to 1Gbit devices

This is a red-herring, for the reasons I've outlined above. With 1+ Gbps connections on your LAN, your L2 network is an order of magnitude faster than your WAN upload. It cannot be the case that a fast LAN makes a slow WAN slower. This is not RF impedance where step-transitions cause reflections; we are dealing in packet-switched networks, where queuing theory controls.

TL;DR: please try OpenWRT QoS instead

I find that outside of chasing diminishing returns - the biggest boost to upload speeds generally involves getting service that isn't from spectrum or comcast.

The speeds you are suggesting seem to imply you are on some cable carrier. Be consciously aware that there is frequently fine print stating up to on your meager upload allotment. You may be optimising against unseen forces.

It looks like you put in some effort on tuning which is awesome - and hopefully is helpful to some as a starting point... but from experience tuning is often very unique to each individual setup. That said: kudos on drawing that much out of your setup.