The complaint was always the same. Someone's on a call, someone else starts backing up photos or pushing a big build, and suddenly the call turns into a slideshow. Plenty of downstream bandwidth, no obvious problem on any speed test, and yet the experience was rubbish at exactly the moments it mattered.
The culprit was the upstream buffer. A fat upload fills the modem's queue, every other packet has to sit behind it, and your latency goes from 15ms to 300ms while the buffer drains. Bufferbloat, basically. The fix isn't more bandwidth, it's letting the router manage the queue instead of the modem.
On the OpenWrt box I turned on SQM with the cake qdisc and set the upload ceiling to about 90% of the real line rate. That last 10% is the price of admission: you give up a sliver of peak throughput so the router, not the ISP's modem, owns the queue and can keep it short.
config queue 'eth1'
option qdisc 'cake'
option upload '18000'
option download '70000'
option qdisc_advanced '0'
The numbers are in kbit/s and they're deliberately under the measured rate. That's the whole trick. Calls stopped stuttering, uploads still finished in roughly the same time, and nobody has shouted up the stairs about the wifi since. A good evening's work for ten minutes of config.