I came to help but you covered pretty much everything I can think of. However, I do have a couple suggestions to add.

I have 6 cats. My most recent addition was my hardest yet. It took 8 months for our home to balance out enough when they can be out together unattended. We separated our house using baby gates and a metal screen (the cats chewed through the plastic one lol) so they could see each other. We then rotated our problem cat so the scents could mix.

Vertical space and/or shelves.

Make sure to have multiple food bowls in different areas as well as multiple litter boxes in different areas

Another thing you can try is feeding treats/food or playing with them near each other so they start to associate pleasure with that cat around. This can be done with the screen separating them at first. Once the cats start to show more confidence while they're playing and not looking around at the other cats that they're scared of, then you can slowly try to have them play or eat in the same area. If one of the cats causes an issue then just pick them up put them on the other side of the screen for a time out. And retry again when you feel they are ready.

To piggy back on Brave's comment, be sure to play with them all. Together as well as individually to help avoid jealousy issues.

I also have seen no evidence personally that those pheromone sprays and plug-ins do anything.

I understand how you feel about rehoming. I don't condone rehoming for stupid reasons, but sometimes we have to. I will do everything I can to avoid rehoming, but with that said, I have had to rehome 2 kitties. I made sure to know who they went to and I have/had (one had since passed away) regular contact with them and can visit whenever I want to thankfully.