I'm not convinced the issue is with the wi-fi. It's connecting at full strength. I suspect it may be one of the switches but I don't know how to test that.
Not easily is the unfortunate answer, especially if the switches have no reporting interfaces. You would have to rather primitively test each hardware route through the switch, but the effort involved is such that I'd just get a better switch. Not helpful I know.
I'm not convinced the issue is with the wi-fi. It's connecting at full strength. I suspect it may be one of the switches but I don't know how to test that.