I have used ManicTime for several years, and I think it's the best time tracker on the market. There is one thing, however, that continues to frustrate me.
The way the program works is perfect. However the amount of UI interaction required to do things is too high. For example:

When allocating times:
- right click on the time block
- choose 'tag activity as'
- select activity (if on list, otherwise click 'add tag' and type text)

this may have to be repeated for 10 or 12 activity blocks suring the day

Likewise, the timer feature is great in function but too cumbersome to use. If the phone rings during a task, I have to
-click 'stop'
-click 'start'
-type to filter then select a tag

This is just a little too much friction, hence I just don't use the stopwatch.

My suggestion is a 'quick select' bar: I have mocked it up in the image attached. The idea is to have a row of buttons, where only one is selectable at any time.
The 'SELECT' button on the left is special and permanent. When selected, it requires the user to select a job just as occurs now.
The user is able to select jobs to appear as buttons in the rest of the bar. Perhaps the rightmost slot could be a drop down scrollable list of all the remaining buttons in last-used order.

When a job button is selected, eg. 'HTM', then any block subsequently selected is automatically tagged with that job. If an area is dragged, it is tageed with the job (although we need an 'Undo' button - say ESC - for exploratory dragging, or a keypress to temporarily suspend autotagging).

Is timer mode, if any of the buttons is selected, then the timer automatically stops and restarts assigned to that job without any further UI interaction, so changing timed jobs as a single click operation.

If you could make this change, then along with cross platform ability and the server, I think this would become the 'killer app' in the space. 'Timing' for OSX is a serious competitor, but they are at present confined to OSX. If you went DotNet (if not already) and fully cross platformed this with Xamarin (my choice of toolset), or perhaps as an alternative used Flutter, you would be unstoppable.