Recent Ethernet speed issues - RBR850

My wife and I both work from home and do several video calls per day, often at the same time. I previously had an RBR50 system (wifi 5) that I had used for years and had zero issues with for wired/ethernet. We did start to have some wifi issues I believe due to device saturation so I upgraded to the RBR850 (wifi 6). We have Spectrum 1G service (1Gbs down, 35-40Mbs up) working from a DOCSIS 3.1 modem (EN2251) with a 2.5g port.

This setup has worked rock solid for months, at least since September of last year. Both wifi and ethernet speeds have been solid as well as both my wife and I's video calls. Then a few months ago we started noticing some stuttering on calls, video cutting out, etc. We'd get warnings from the vid call apps (Meet, Webex, Zoom, Teams) that we had low bandwidth. When we first installed the wifi 6 router, I had checked speeds from wired and wireless devices and consistently got 900+Mbs on wired and 500-850Mbs on wifi (for wifi 6 capable devices). I still get excellent wifi speeds all over the house. However, now when I run the Ookla speedtest on a wired device like my desktop, it starts off as high as 600Mbs but drops to 200-300Mbs as the test runs, almost like it is saturating as it tests, even though the test is like 10-15 seconds. I tested this with our Windows and Mac laptops as well with the same results. Previously when I'd run this test it would start in that 600-700Mbs. range but get FASTER as it went, ending at 900-950Mbs down. So I immediately ran through several troubleshooting steps:

• Tested speed on router under Advanced-Setup - 1150Mbs down, 41Mbs up
• Connected desktop direct to cable modem - speeds of 950Mbs plus down, 35Mbs up. Just like it used to be and slower due to it only being a 1G NIC I assume. This leads me to believe the issue is the Orbi ethernet router.
• Rebooted everything - no change
• Downgraded firmware - no change
• Switched ports for attached devices - long shot but no change
• Upgraded cables to cat 6a STP 10G - no change
• Disabled several features one at a time - NAT filtering to open, SIP ALG off, WMM off (I know it's wifi only but tried), UPNP, etc. with no change
• Checked Network statistics on router page - Ports 1-4 solid 1000M/Full solid over an hour or so
• Disabling or turning off all other wired devices and testing a single ethernet port - no change
• Checked all attached devices - nothing

I'm at the end of my rope. It's been suggested to invest in a stand alone wired router which if I have to I may try but given this setup worked flawlessly on the previous Orbi and this one for several years, it sure seems like it is fixable. Can someone please help me diagnose this issue and hopefully fix it?

Thank you in advance for your help!