I've had the same thoughts, about funding them and about how to design them so that multiple people can win. Perhaps there's something there with NOSTR, I haven't set mine up yet but will soon.
I've thought about maybe a system where a slow release of smaller puzzles that each lead to a clue, and a reward if you're the first to solve it. The small puzzles can be dropped at regular intervals (blocks?) so people know when to look for it. They could even be made difficult, such that multiple puzzles are active at a time. The clues could then lead to a larger puzzle with a bigger payout and be (potentially) unsolvable until each of the smaller ones are unlocked.
Again, doesn't really solve the long-term funding issue haha! But it's a fun hobby for now
A problem with multiple payouts per challenge are sybil attacks:
The first winner can create multiple identities and thus win the challenge multiple times if something like LNURL-auth is used.
Maybe I am overthinking this and I should just build? ... lol