I think this is difficult with the 7-day reward rule.
For posts older than 7 days

As an alternative method,
We propose to manage points for sponsorships.
Pre-purchase points with HIVE, HBD, and sponsor with points.
(may be cumbersome)

Also, as an alternative,
Have a reward pool with 5% reward for posting,
When the user clicks the donate button, the daily reward is determined within the reward pool limit and divided by the number of donations. (Only allow donations made through the homepage)