Tracing the other direction is often a useful exercise. Based on the data I saw of yours, though, you're probably not going to learn a lot new about your problem from tracing outside-in. The further from you the congestion appears on the route, the more useful tracing the other way (the "return" route) is. In your case, the problem occurs very close to you. Without trying it, though, you won't know for sure if it exposes some useful information or not, though.

Some routers can forward ICMP requests coming in at their NAT boundary, but usually it's easier to use TCP and forward that (which should be possible).

Even easier - you can sometimes target the internet-facing IP address and get a response from your router / modem / device. You might know this address, but if you don't you can go to someplace like whatismyip.com to find your internet-facing IP. If the latency / congestion appears at that point, then forwarding packets inside your network is not going to add any latency or much insight to the problem, so I'd always suggest trying this first. You might not get a response, though, and then you may need to forward to a device inside your network that will respond.