A blackhat link building tactic I'm fighting

I know that the standard advice is "don't bother trying to fight blackhat links, Google will catch up to them" but in this case, I'm not so sure Google could ever find/penalize these links.

The tactic a competitor is using - reverse image searches a photo in the public domain (government source) or from Pixabay, and blasts out emails to those domains asking for their website to be attributed as the source. For all I know they aren't even claiming to own the photo, just saying "hey I see you're using this image, could you link to my site as attribution?"

This site has gotten over 250 linking domains in 2020, over 60 above DR50 including really high-quality websites that would never link to them otherwise.

The site is ranking well and continuing to rank better every month. The images are niche-relevant so the linking domains are typically niche-relevant as well. This is the only linkbuilding tactic they're using.

My ideas for fighting this tactic...

1. Email the sites (starting with the highest DR) explaining the situation and asking them to remove the link. I have had 90%+ success rates so far, so this can also be useful for warming up your inbox/keeping it out of spam as you get a lot of people interacting with you. It does take time and could be hired out to a VA.
   
   
2. Take legal action. This probably isn't even possible, I would need the actual copyright holders to be the ones to take action, and even then, it might be more of a grey area. I don't know if the site owner is even American because they use a fake name so this isn't really worth pursuing.
   
   
3. Tell people about this tactic so it becomes well known and overdone and stops being useful.
   
   

Thoughts?