Either implementation would require changes in how the client sends out notes, the amount of effort (programming wise) is the same.
The primary benefit of the relay method however is it means clients can comply without falling afoul of Apple and Google rules. Most people use this on mobile remember.
If all relays can provide nsfw content, devs will be forced to block the nsfw event type completely.
If the feature is instead a neutral relay selector, well that's perfectly innocent...
You make a strong case wrt OF models. But then I'd also argue if you encourage pros only you narrow down your audience significantly because your relay or classifier or whatever just becomes an ad channel.
A lot of people are sick of FetLife now for exactly this reason. There's not much community element anymore. Most of the active women on there are just shilling their OF pages.
I assume it works if they keep doing it, but with Nostr you'd have to opt in to nsfw content in some shape or form. If after doing so your feed is just flooded with OF ads, I'd put it to you most people will just turn it off again.
After all Pornhub is free, Literotica is free, LLMs trained to act like kinky AI gf's are free...