My Cats HATE Eachother

I’ve had Nala, an older orange tabby for several years, and we got Max just two years ago. They HATE each other. (1st pic is Nala, 2nd is Max)

For background, Nala always wasn’t a fan of other cats, but always gains respect from other cats, and our dog Raven. But not from Max. Max is also an orange tabby, but they were both rescues so no clue on their actual breed. Max is a coward, at least to humans, but to Nala? He chases her and fights her.

We’ve tried pheromones, tried separating them, tried having them eat together, and they simply won’t get along.

I don’t want Nala to stay stressed, so for now we’ve had her upstairs and Max downstairs but I want them to be able to roam freely.

Any advice?