Why your ping spikes in the evening
Most of the time it is not your ISP at all but bufferbloat on the uplink. Turning on SQM (cake) on the router and setting the shaper to about 90% of the measured upload speed fixed it on my cable line.
Mesh Wi-Fi: wired backhaul is worth it
Moving two satellite nodes from wireless to Ethernet backhaul cut the latency jitter in the back bedroom from 30 ms to under 3 ms. A flat cable under the carpet was enough.
Measuring latency the boring way
A small cron job that pings three targets every minute and writes the median to a CSV tells you more than any speed test website. Plot a week of it and the patterns show up quickly.