Separating different websites is more general, you should be able to find plenty of resources how to separate the processes: How To Host Multiple Websites Securely With Nginx And Php-fpm On Ubuntu 14.04 | DigitalOcean
And with the jails in BSD: https://ramsdenj.com/posts/2017-06-05-nextcloud-in-a-jail-on-freebsd/
I hope it will be in the far future and you’ll have enough time to get more familiar with this topic. If you have critical setups, you shouldn’t rely on community support forums and consider professional support.