So the USB-to-serial adapter arrived. I can now program the ESP-01 module, considerably smaller than the development boards I have: the WeMos D1 mini, for example, is a little too big to fit inside the small space available in the plastic enclosure.
With a bunch of jumper wires I kinda managed to connect it to an ESP-01 module. Did a first programming, then I unplugged the wires from the adapter, re-plugged the ones for the fish, and checked if it worked. It did work, but I needed a few adjustments. Unplug again, re-plug again. And every time, carefully check for mistakes.
Mildly fed up by this tedious process, I decided to build myself an adapter board. So I could plug the ESP-01 on one side, and the USB-to-serial adapter on the other. An evening and a morning later, here's the result: