I also am not really a fan of how the ratings work at the moment, and I think there is a lot of confusion in the community about it, why ratings go down with a win being a regularly asked question.
That said, just to comment on a few things
I think you’re focusing too much on a case where both players have a stable rating/rank. Lots of players improve rapidly, and they can play much more consecutive games online than they can over the board to facilitate this.
Not only that, it wouldn’t surprise me if for instance one player could have three different ranks depending on whether they play blitz, live or correspondence games. I expect I am much weaker (many stones) at blitz than live or correspondence, in part because I either time out (10s default byo-Yomi is a bit too fast for me) or have to play moves I can’t even read one or two responses to. OGS merges all those ratings into one overall rating (I know it has those separately as well) and I don’t think it’s surprising if that would give big rating fluctuations.
I think there was some issues with a volatility parameter and how handicap games were treated in the past. It was also suggested still some issue to do with handicap games that I hope was reported
One could argue to split off the other side boards and other time settings from the overall rating, which was discussed here with data