I'm curious to see what others use, but I've been using Uptime Kuma for this purpose at several different sites I manage. I run Uptime Kuma on a VPS and locally on site. I have the local instance monitoring the router, gateway, DNS, and several other internal and external devices. I also have the local instance do a "push" check-in on my VPS instance. This gives me a pretty holistic view of things.

Maybe smokeping?

https://github.com/linuxserver/docker-smokeping

I use Uptime Kuma as mentioned in another comment, but also Speedtest Tracker which makes automated speed tests at the interval of your choice and displays them in a coherent dashboard. It’s pretty neat and it did help me figure out a few things about my network.

For a while now I've had Grafana hooked up to InfluxDB and Telegraf. Using Telegraf I setup pings to ips along my route to the larger internet, major dns providers, and several large internet sites. I measure response time and packet loss. It has allowed me to cut through the Comcast BS when diagnosing problems with them. I can tell them for sure that the problem is inside their network and is the X hop from my router.

I recently started setting up Grafana over on a different server and I'm using Prometheus instead to monitor more than just the other server I was monitoring. I haven't yet set it up with that but it looks like something similar is possible with Prometheus based on the small amount of research I've done on it.

Not sure, but I think that designing an internet measurement in a RIPE Atlas network might just fit this task? https://atlas.ripe.net/probes/ You have micro cuts, but are those only to the big name websites or to something local as well? Might help answer that, and give ISP data on where exactly are they hitting the bottleneck and what are they missing monitoring.