BobMc, did you ever get this fully answered / implemented? I know it’s been many months from your last post; I am also a Spectrum customer with this is exact modem (EU2251) & I do use DOCSIS 3.1 compliant 2.5GbE MoCA adapters (with a PoE / MoCA reflector filter at the head in, not at the modem; as if you have access to the entry point, it’s better at the head in point, as the filter / reflector <I used a Belden which is also a “reflector” as well as a PoE / MoCA filter>, will also preserve the integrity of the signal to the other MoCA units across all of your COAX when placed at the entry point ahead of any splitters; as well as protect your network from the signal bleeding out to neighbors of course.. Some might be filter only, on that I am not clear. I did also upgrade any splitters (I only had 1 also at my entry panel) to 2.5GHz splitters, & lastly I upgraded ALL COAX wall plate or in junction box “couplers” to modern 3GHz COAX (blue plastic insert instead of the traditional clear white plastic) couplers, & it ALL works REALLLLY well. I have used it with Spectrum @ 1Gbps speeds inbound, & as an FYI, by using Cat-8 braided / shielded Ethernet cables everywhere (not Cat-6), I also gained the over-provisioning Spectrum pads into the connection. I also only really needed 500Mbps inbound ultimately, so later dropped from the 1Gbps to 500 like you are (or were) at. BUT I actualize (using SpeedTest, but always the SAME testing server each time I test, important detail); but I actualize around 580Mbps inbound consistently on the 500 package.
If you did not embark on the project in the last 6 months & still have questions; since I have a very similar setup, I would be happy to share any details that might be of help. As well, the compliant MoCA adapters I used came from TranLite Global, model # TL-MC85, & you do have to log into it’s WEB admin & tweak the band settings & LOF value to make them DOCSIS 3.1 savvy, but they do provide the documentation for that (& it’s really just a couple of clicks & typing in the updated LOF value & clicking save / reboot). But everything works beautifully, & no service conflicts with the modem / video etc., & now the COAX is all 2.5GbE as these units have both an onboard pass thru COAX port (eg 2 ports vs 1, which helps reduce db loss if you use a 1-port only MoCA adapter), & it has both 1Gbps & 2.5Gbps Ethernet ports.
FYI..
Randy