Good ideas and interesting project you have going.

I was considering a secondary form of hashtags: funded hashtags.

Funded hashtags would be funded with let's say 10 sats. This would act as a cost barrier that reduces spam. Perhaps the relays that pick up notes with funded hashtags could gain a percentage of the sat funding.

While I hate pay-to-win features, spam will become an increasing problem and if the cost for funded hashtags is not expensive it might work.