I would shell out a few bucks for it that is for sure. There is a business incentive. Maybe they could do an incentive for the pro subscription - get a few bucks off the app or something.
A work around that I am doing is just to actually name the task with the preceding Letter / Number and then sort by name. works ok, and it keeps your tags free.
When you are in the task list, top left of the list in small letters it should read Hidden Tasks. touch that and it will open up a filtering section. under filters there is a section called completed tasks and you can toggle what you want to see.