I am okay with any of them. They all sound great. Maybe except the part where you can only buy reserves instead of earning them.

Give people options. Let them have the ability to earn reserves.

Any worries about making it complicated just use a simple projection calculator.

Give them a calendar and let them pick a day to stop doing stuff. Then show them how long they can go without doing stuff until they lose streak.

Have this feature regardless the underlying calculation, then is fine.

Don't make the users think.

Also follow stackoverflow way of showing them based on their streaks they are top x%. Rather than showing a pure linear rank by raw streak.

This gives people a chance to feel they can move up

See attached