I just went through this in January. We have similar equipment - UDM Pro, Switch 24 PoE.
TL;DR: I got pulled into the cable modem upgrade you’re considering. I didn’t go as deep as you’re considering going. I think it’s a waste of time and money.
I had a Netgear CM1150 which Xfinity started telling me was throttling my speed and no longer supported. I’m paying for the 1.2Gbps tier and while the CM1150 is functionally capable of supporting it up to 2Gbps, it Xfinity only “certifies” it for 800Mbps. They’ve done similar things with other self-owned cable modems. I’m not sure if the they are actually throttling speeds to “certified” limits or if that’s just part of their strategy to get people that own their own gear to move to the Xfinity “gateway”.
I don’t think we need to discuss any more about why @MamaTried, @Johnny_Ooooops and I don’t want to use the Xfinity gear (e.g., Home Hotspot).
I actually fell for the whole thing about 1.Gbps without thinking it through the way you are.
We moved last year from an area serviced by Ziply Fiber (derivative of FIOS) to an Xfinity-only area. Ziply gave us bi-directional 1Gbps. That’s an amazing service tier - I miss it. Had Xfinity over there, too, in parallel. Every once in a while, I re-seed my offsite Azure backup and it’s amazing to watch it fly at a legitimate 1Gbps upstream. I still haven’t quit wrapped my head around being back in the world of asymmetric speeds and so when Xfinity started telling me that I was being throttled, my thinking got a little muddled and I bought a new cable modem without thinking it all through.
I did the same research you did and bought the Motorola MB6811.
I’ve owned my own Xfinity gear for many, many years and am no stranger to cable modem upgrades. My experience has been that they are mostly self-service now. The MB6811 update wasn’t - was one of those hours on the phone experiences. They just couldn’t get it provisioned. They finally did.
And that, my friend, is where my story ends. Because I realized, like you, that the next step would be to try to connect my UDM Pro to the Motorola using the WAN SFP+ port and an adapter. If I were to get that working, I would have essentially raised my downstream bandwidth capacity between the UDM Pro and my UDM Switch (a USW-24 PoE) from 1Gbps to 1.2Gbps. And that would accomplish absolutely nothing. Because I don’t have a switch congestion problem. Neither do you. If I did, then the Xfinity upgrade would be an orthogonal problem. On top of that, my UDM Pro has never once measured my Xfinity downstream speed at greater than about 900Mbs. I suppose that could change if I had a faster interconnect between the UDM Pro and the cable modem, but I seriously doubt it.
You’re considering going deep. I wouldn’t. It’s just running in place until you decide you want to solve for higher bandwidth wired connections to your client machines, which you might never decide.
Maybe upgrade your cable modem if it’s throttled to 600Mbps but consider stopping there.
I’m curious what you’re talking about with port swapping, though. You can see how I’ve got my switch cabled to the UDM Pro. Are you already doing this or are you doing something different?
Also…look how nicely that RS1221+ pairs with the UniFi gear