From the Google WiFi app I can measure the bandwidth to the connected wireless devices. Is there a way to achieve achieve the same measurement in OpenWrt?
How sophisticated to you want to be?
A streamlined method is to use a file transfer application that gives a running speed.
See this wget screenshot:
It's easy to check the negotiated raw radio speed in the AP's client table. It is reported on the main status page under Associated Stations. Actual usable throughput would be about half of the raw modulation speed.
It is unclear if the Google system checks anything beyond that. Sending a timed blast of data to a client would require an application on the client itself to return it. In open source there is iperf3.