i think pow is easy to implement without significantly affecting UX.
if default difficulty is something like 10, no users are gonna notice, but spam reply bots suddenly require a lot more resources 

 yea that was also my thinking, PoW won't help for one-to-many spam (as the work can be amortized over many targeted users), but for mass DM/reply spam it might, a bit
 

 yes, and one-to-many spam is where a ‘web of trust’ can be applied without any onboarding trade-offs