Is it possible to do a speedtest at each satellite in addition to the router?

You could attempt to run a speedtest wirelessly when connected to the Orbi router and to each satellite, though you may find it difficult to ensure you're sonnected to the one you want to be/think you are connected to, since, frustratingly, devices don't always just connect to the nearest AP.

If you have a laptop with an ethernet port, you could connect to each device in succession via ethernet, run a speed test, and then repeat for each one. This will ensure you're connected to the router/satellite you want to be connected to and think you're connected to. But you'll have to run the test either from a website or app on your computer, not on the Orbis, since only the router has a built-in speeed test functionality. And, for consistency's sake, you may want to run even the router test by the same means as you test the satellites, just so you're comparing apples to apples.