The smart thing to do is to regularly keep your manager updated on what you're doing, especially if they don't come by regularly and ask you.
Especially if you are WFH.
Code is often improved by removing code.
Task completion would also be abused, because it's too vague. It simply shifts the burden to the one formulating the task (e.g. preventing holes in the specification like missing performance or hardware requirements).
Exaggerated example: if the "task" is to automatically deliver a report containing certain data and formatted in a specified way, the easy way might be an implementation that stalls the DB server for hours with deeply nested FULL OUTER JOINs on non-index fields.
The task would be completed quickly and arguably correctly, since neither runtime nor memory requirements were explicitly specified...
But you said it yourself, any system is going to be abused...
Out of sight, out of mine <== don't let that happen to you
I don't believe it is any coincidence that the highest compensated engineers I know also are highly visible through their own efforts.
Meta-metrics might be much more helpful, though harder to come up with, quantify and monitor. Things like defect rates, user reported incidents, user satisfaction, stuff like that.
Things that cannot easily be gamed from within the development process and that are still directly linked to the success and economic viability of the product and its development methodology (though on a higher level).