I disagree with this. The “Edit filter” dialog is for editing the current filter. It’s purpose, in contrast to the filter sidebar, is to fine-tune the current filter. It’s intended as a more advanced way to filter the results. See also: Edit filter dialog redesign
I don’t really follow that the Edit filter button is unclear in its intention. I don’t really see how it would infer expanding the existing options, but perhaps I’m misunderstanding what you mean.
The initial view that most users will be introduced to will be with the sidebar open - assuming they are running on screens that can accommodate it.
I do think that the sidebar toggle button in its current incarnation is the main source of confusion here. I agree that users will likely be confused by the actual action of the button vs the implied function. I disagree with the apparent function of the Edit filter button, and I’d want further data on that.
I suspect that the solution to this will involve re-solving how to toggle the sidebar and the current filter button becoming edit filter button, which will maintain consistency for existing users. I’ve been prototyping other ways to toggle the sidebar and haven’t settled on anything suitable so far.
Onto your proposed solution: I’m still of the belief that the + symbol is as misleading as the current implementation. I’m also pretty unhappy with how it looks when there are no current filters applied:
This will be the initial view that users are presented with, and I don’t think it improves clarity over having a specific Edit filter button.