You should consider hiring an online reputation management service.
Such services really are SEO agencies with a very narrow objective: to bury negative pages.
That's all they do.
If you were to try to come up with your own SEO-based attempts to rank positive reviews of your service or business, chances are, it's going to take too long or you're just going to fuck it all up.
I would suggest you hire a specialist to take care of this problem.
It won't happen overnight, so don't get your hopes up.
But eventually, you will see the ranking of that negative post sink, especially if the ORMS is using higher-quality parasite pages.
Remember, the reason why your competitors' negative post is ranking is because that page is leveraging the high SEO equity of the Reddit domain.
When you hire a high-quality company or you target high-quality parasite platforms, you can suppress or at least push the negative post to the second page of Google.
Let me tell you.
Once you get that done, you've won.
Why?
You can hide a dead body on the second page of Google because people rarely check.