The 5M+ Donator was for when RSGP was able to be donated back in the day. There was previously a way to donate RSGP as well, where it would be provided to a third party, sold, and the money forwarded on to Matthew.
With the new donation center, I believe users are now able to choose the old or new ranks depending on their current level.
My best advice to you would be to sell RSGP using a trusted middleman and then donating that to Sythe if you desire.
Best of luck.