Cloud streaming unusable — latency and packet loss appear one hop past my gateway, need help narrowing it down

I've been having significant problems with cloud gaming — the stream freezes for a second or more at a time, several times a session, badly enough that it isn't really playable. NVIDIA's own GeForce NOW diagnostic scored my download path 0 out of 100 (in their report that category measures latency and buffering on the path from their server to me, not raw speed). low-bitrate streams seem fine, while high-motion ones are significantly worse. My speed is also fine, so I did some testing to figure out what else it could be. Here's what I found, and what I think it might mean, but I'd really appreciate help from someone who can see my line telemetry to narrow it down.

Setup: Xfinity XB6 (Technicolor CGM4140COM), MacBook Pro on 5 GHz Wi-Fi — RSSI -44 dBm, noise -95 dBm, 866 Mbps link rate, 802.11ac, channel 157.

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Test 1 — Three pings run simultaneously under identical network load

I wanted to see where the degradation starts, so I saturated the connection and pinged my gateway, a Comcast hop, and an external address all at the same time.

Gateway (10.0.0.1), 120 packets: 0.0% loss, 6.7 ms avg, 29 ms max, 3.3 ms jitter
Comcast Burien (68.87.206.157), 120 packets: 9.2% loss, 82 ms avg, 172 ms max, 32.7 ms jitter
Google (8.8.8.8), 258 packets: 11.2% loss, 74 ms avg, 285 ms max, 47.6 ms jitter

A separate 300-packet run to my gateway gave 0.4% loss and 7.6 ms average.

Test 2 — Traceroute, idle vs. under load

Idle: hop 1 at 3 ms, hop 2 at 13-16 ms, backbone hops 13-18 ms. End to end about 16 ms.
Under load: hop 1 stays at 6-7 ms, but hop 2 (100.93.165.194/195) jumps to 84-111 ms and hop 3 to 76-104 ms.

Test 3 — macOS networkQuality, eight consecutive runs

Idle latency: 31 ms. Under load: 237 ms, with a 502 ms worst case.
Downlink capacity: steady at 78-80 Mbps every run.
Uplink capacity: 9.0, 21.3, 9.0, 10.2, 19.5, 8.9, 20.8, 21.5 Mbps.
Also reported: L4S disabled, ECN disabled.

What I think this points to

The part I feel confident about is that it isn't my home network. My gateway returned 120 of 120 packets under 30 ms with almost no jitter while the link was fully saturated, and my Wi-Fi signal and link rate are at or near their maximum. The degradation consistently appears one hop past my gateway.

Beyond that I'm less sure. My guess is that the sustained packet loss isn't just congestion, since congestion usually delays packets rather than dropping 9% of them — and the upstream capacity swinging between 9 and 21 Mbps while downstream stays rock steady seems like it might point at something on the return path. But I could easily be reading that wrong, and I don't have visibility into anything past my own modem.

What I'm hoping for

I'd really appreciate someone on the team taking a look at my line and helping me narrow this down — whether my interpretation holds up, what the telemetry actually shows, and where to go from here. I'm happy to run any additional tests that would be useful, at whatever times of day would help, and I can share the full raw output from any of the above.

One other thing I noticed, though I'm not sure how much weight to put on it — networkQuality reports L4S and ECN as disabled on my connection. I don't know whether that's just the default state on my end or whether it reflects something about the line, but I mention it in case it's relevant. It might be a piece of this, or it might be unrelated to the loss entirely.

This has made cloud gaming basically unusable for me, so any help getting to the bottom of it would mean a lot.