As you’ve discovered, this kind of calandar-bound goal is difficult to beemind.
Some people use a daily goal to ask ‘have I done enough today?’, entering a 1 on days like Tuesday when ‘enough’ is ‘nothing’. If you care about the total times and have access to custom goals, then the badly-named aggregation method of binary will treat 0 as 1 for the purposes of staying above the line.
What I’d probably do is to make three goals, one each for task-monday, task-wednesday, and task-friday. Annoying as it might be to have separate graphs, it’s much easier to set a goal up to recur on the same day each week.
As you point out, derailment is tricky with anything more complex than this, but a calendial-style goal can simply have 6 days of respite so that it’ll be in the red the same day next week.