Interesting idea, as far as I can tell evenroute offers a commercial product, the iqrouter, based on this approach (not identical in all details). You might want to have a look at https://forum.openwrt.org/t/sqm-autorate-ingress-can-i-set-thresholds-for-this/55771 where @dlakelan developed something functionally similar using erlang...
This is pretty intrusive, as you will have repeated outages due to your schedules measurement runs...
This is pretty sane, you need your own measurement endpoints to get halfway reliable speedtests, but keep in mind that you will measure the whole network path, so if the transit/peering between the network housing your automatically adjusted SQM instance and the AS/datacenter housing the measurement endpoint is congested, you will throttle all outgoing traffic, even if destined to non congested networks.
The downside is, that when your ISP just throttled you from 30 to 14 you will need to endure quite substantial bufferbloat until your adjustment script is run again.