Dating website, experience in digital ocean, optimisation, configuration and WordPress lamp stack with Ubuntu 18.04 and MySQL 8

hi i run a very large dating website about 700,000 members and has been installed on digital ocean i upgraded my package to However my website fails to load properly and gets error 502 page errors, i had the server optimised and all images but still this problem persists my website yesterday suffered server issues and wasnt online i am currently locked out of wordpress admin because of this and have no access to wordpress admin however there is acces via digital ocean sever through command line i have been searching for a concusion and believe Installing the fastest WordPress lamp stack with Ubuntu 18.04 and MySQL 8 as my site is running on apache and is unable to cope with the data from what ive been told. looking at error logs possibly would confirm that apache is not upto the job im looking for someone who has the experience in dealing with digital ocean and knows how to set up and install configure and optimise my wordpress site so it is on the fastest setup available. my website ultimatly needs to be able to perform well in the future with handling the data of in excess of millions of users and in them being online at the same time so i need to be on the best software that can handle this. i have no experience in digital ocean or software so that is your job to get me the best performance available with your experience.