But it still requires the official mobile app for the initial login. When I used it, logging in once seemed to be good for a few months, as long you open it from time to time. There’s also a Whatsapp clone that seems to support login, but I haven’t tried it and it hasn’t seen updates in a couple years: https://github.com/KhubaibKhan4/Whatsify-Android
Is there, by any chance, an alternative client?
There’s the Whatsapp web client on F-Droid: https://f-droid.org/en/packages/io.kuenzler.whatsappwebtogo/
But it still requires the official mobile app for the initial login. When I used it, logging in once seemed to be good for a few months, as long you open it from time to time. There’s also a Whatsapp clone that seems to support login, but I haven’t tried it and it hasn’t seen updates in a couple years: https://github.com/KhubaibKhan4/Whatsify-Android