Strikethrough of Checked items in Checklists is an extremely common method of indicating “done”. Another option might be to “gray out”/“fade” the text, but strikethrough is much more common in my experience. Do none of the other tools you use have strikethrough for checklists?
When I searched, many services that support markdown like seem to have similar discussions.
Actually, I wrote the article because an external collaborator complained of discomfort to me, but it seems difficult to ask that person to inject css.
I wrote the entire detailed specification of the task as a checklist, so it seems that the problem was bigger.
I think it makes sense not to write large portions of text as checklist items. But I am curious why you initially decided to do so (even though you are now changing your approach). Perhaps there is some other formatting/feature that could accomplish the goal you had for putting everything under a checkbox?
As one example, which may not apply to your situation, I often use the “folding” capability of Headers to sort of indicate to myself that something no longer needs to be read in-full. This “folded” state does not get saved in the doc so it will not show up for other viewers, I think. But it is really just an example.