How about you 'de-clutter the game world' and remove ashdale and make crassian a f2p boss? I think this is a decent sollution.
I appreciate this isn't entirely answering your question, but Agoroth is a F2P boss, and A Shadow over Ashdale which unlocks it is a F2P quest. :)
Now, it's a safe-death F2P boss aimed at novice players, and therefore is weekly to throttle the rewards. Which I imagine isn't fulfilling your criteria for a F2P boss, but balancing-wise that's how we had to do it to meet the accessibility aim and not release a no-risk highly accessible wealth-injecting loot pinata.
Incidentally, we do see healthy weekly engagement with Agoroth even in its current state. Those black pearls seem to still be desirable even 6 years later.