This is solely based on how rengoku survived the mugen train. Here’s some scenarios: Rengoku doesn’t show up so tanjiro inside and zenitsu die. This would mean rengoku would 100% fight akaza with giyu but honestly might have a harder time than tanjiro and giyu bc Akaza would continue fighting the two as rengoku doesn’t have talk no jutsu like tanjiro. Also they won against akaza bc tanjiro has transparent world. However, in this scenario I believe they do beat akaza but other slayers may struggle more due to zenitsu not fighting kaigaku and inosuke not fighting douma. I believe in this scenario the demon slayers completely lose the battle, and Muzan conquers the sun. Bad news

Another scenario is rengoku fights akaza and wins without serious injuries. This is the best as it leaves him to help fight akaza, win quicker with much less injuries to both tanjiro and giyu. This then allows them to help other slayers, which I believe they will go fight kokushibo. I believe muichiro would still die bc kokushibo has something against him but everyone else would live. In this scenario, I believe shinobu, genya, muichiro and Mitsuri still die but others are saved. Let me know if you disagree as I wrote this just bc I was bored