You are overthinking it, on so many levels...
1. You overvalue the benefit the links on your website have for them. Do you really think that when you delete the links you will overtake them in ranking? I highly doubt so.
2. You are overvaluing their SEO skills/negative personality. Maybe they do not even use any tools like Ahrefs and won't even notice that you deleted their links. And if they do notice, for sure you would not be the first person ever to do something of that sort to them... Domains expire, owners change, articles get added and deleted. They paid you nothing for the links. Do you think they have resources to start some negative SEO campaigns?
If I was you, I'd do one of the following:
1. Delete the links, and forget about the subject immediately.
2. Write to each one of them, saying that your website changed ownership/policy, and that the new owner charges now $50/$100/$500 (you pick your price that makes sense for you an in the niche) for each outgoing ******** link per year. Either they pay within 7 days, or their links will be deleted.
