@DolleDK: I am a Vado 6.0 owner, too. I ride it all year long, also in the rain and during the snowy/icy Winters. I do care about the drivetrain.
First of all: When it was the last time you measured the chain stretch? When it has gone past the 0.5% mark, you should replace the chain. When it has gone past the 0.75%, it might be not too late. If it has gone to the 1% mark, your small sprockets of the cassette are ruined.
Now, if you want to keep it simple and fast, replace both the cassette and chain (it has to be done at the same time). If you can do it yourself or have a good mechanic, you could just replace the 4 smallest cassette sprockets (and the chain, of course), which is a way cheaper than replacing the cassette. Do you clean and lube your chain regularly?
The
only reason for the chain skipping on the smallest cassete cogs is the worn cassette and the chain. Whenever we ride in the rain, we start hearing our bike better. There is a strange noise in mudguards, our brakes sound differently, and we start noticing something's wrong with the drivetrain. I notice my e-bike could be maintained better when I ride it in the rain myself... Being wet, cold, and irritable