They have phone apps for both iOS and Android which work offline. Their iOS app has a "companion" on the Watch too, so you can quickly switch activities on your wrist. I think without this I wouldnt have the patience to even track bathroom time (another thing that I vastly overestimated before I saw the numbers).

I´m still hovering between writing my own tracking system, for more control, and the convenience of a pre-made system. I would most miss the watch app, I think, as I have no idea how to make that.

Possible compromise would be to use this app but automate the data export and analysis in my own custom environment.

But yea, long story short, Atracker, and offline works well.