The over/underexposure indicators are applied to the output of the whole pipeline. See Channel mixer behaviour (and interaction with filmic) in darktable 3.2 - #5 by elstoc
I think you dance around too much. You should adjust exposure to get your midtones right.
Adjusting colour balance will adjust filmic’s input, so avoid large changes there. However, if you change contrast and midtones in colour balance, you can also adjust highlights (which is linear scaling, like exposure) there, too. Also, why use the tone EQ for ‘general brightness’ – the whole idea is that you adjust specific areas independently (at least if you use detail preservation).
For colour balance: create a black-to-white transition in the Gimp (even better: posterise it in the Gimp, so you get steps that show up distinctly in the histogram); load it into darktable, and see what colour balance’s highlights (gain), mid-tone (gamma) and shadow (lift) controls do.
For contrast EQ, I use a trick of applying a parametric mask concentrating on the mid-tones; that way, highlights and shadows are usually not altered that much. Also, I leave a bit of safety margin in filmic.