CM1200 Modem snapshot w/ Nighthawk R9000 router - topping out at 600Mbps hardwired, pls help!

You'd think their system would flag unsupported modems when you call in and they pull up your account and they'd say something about it before sending a tech out.

Any connection instability/glitches or do you just get a smooth 600-700 all the time?  Your downstream power levels are on the high side, but signal-to-noise and error rates look good.  The OFDM channel is supposed to have a lot of correctable errors, so don't worry about that.

The CM1200 could support the 1200 plan, but it still might not work right.  Comcast sends out a settings file to the modem, and you might not get proper settings if your speed plan isn't supported by the modem, hence the unexpected results BruceW was talking about.  The CM1200 does support link aggregation, and it might actually work properly since you have a Netgear router that supports link aggregation and even has a 10Gb SFP+ port.  But it will never be officially supported at 1200 by Comcast.  Comcast won't officially support anything over 949Mb on the CM1200 since they require a single ethernet port capable of reaching full plan speed.  Link aggregation is often flaky and causes problems if you don't pair up the right equipment and I sure wouldn't want to deal with all the problems it can cause if I were running a consumer ISP.  2.5Gb ethernet is cheap enough at this point.

Sadly I think you're going to need a new router to get full speed on a 1200Mbps service plan with a supported modem.  A supported modem will have a 2.5Gb ethernet port and so far I haven't seen one that supports link aggregation.  Looks like your R9000 is a high end model from 2017 and doesn't have a 2.5Gb WAN port, so it'll top out at 949Mbps when paired with a modem that supports a 1200Mbps plan.

You'll just have to decide if you want to buy a bunch of new gear or lower your plan speed to something yours works with.  You might consider renting a gateway for a month or two to test with.  That way modem problems are Comcast's problem.  Then once they sort it out buy your new gear and return the rental.  You can put a rented gateway in bridge mode and keep using your router.

If you decide to buy new gear I'd suggest checking the full list of supported devices linked from https://www.xfinity.com/support/devices/  It's a .pdf file. They don't seem to have updated the web page with the Hitron modems that support increased upload speeds.  Right now there are only 3 models that do- Hitron CODA, Hitron CODA56, and Netgear CM2050V.  With a 1200 plan & no Comcast voice service (I'm assuming you don't have it since a CM1200 doesn't support it) you're probably looking at a CODA56.

They seem to be slowly adding modems to that list.  At first it was just the two Hitrons.  Then more recently they added the Netgear CM2050V, which is what I use.  I have a Comcast phone line.  Last Saturday I woke up to no internet service.  After a little investigation I realized my CM2050V had crashed.  I couldn't even bring up the admin page.  So I power cycled it and everything came up as usual... or so I thought until I checked the modem admin interface.  I had an upstream OFDMA channel locked, so DOCSIS 3.1 upload.  I haven't gotten the increased upload speeds yet, but having an OFDMA channel means the mid split is done on my segment and it's coming.