Unless you've replaced the pulleys, those are both ball bearing type bearings, not bushings. So not sure what could be squeaking.

You shouldn't have filed anything. That washer kind of snaps onto that spacer. That spacer is split and needs to be compressed a bit to get the washer to fit. Now that it is not compressed, that could have something to do with why it is squeaking.

Are you certain it's the RD and not the chain or a cleat or something?