A lottery or “gacha game” type mechanic could be a fun way to burn up some alchemica, something like this could also replace (or preserve) the raffle events that people have enjoyed for a long time. Keeping it self-funded as you’ve suggested would also help keep it sustainable, although I think there would still need to be extra incentives (perhaps game items?) seeding the pool to keep people interested.
The need for sinks is definitely a concern as the economic system gets worked out. Check out this thread for some more ideas, and feel free to add more of your own: In-Game Alchemica Sink Suggestions