So, about this. Eventually I gave up and let the watch update without saving the data. Then Suuntolink worked for a while, though the app remained useless (could connect but synching reliably crashed the app).
Now Suuntolink has stopped working. It will tell me happily that my “watch is up to date”, but it won’t synch anything anymore. Ever.
I’m nearly at the point where I think I’m just going to throw it out and “write it off”… This is way too much hassle.