Unfortunately it is difficult, Magento is a very complex package and it is difficult to find support.
It is a high probability that one of your extensions (any of them, not just payment ones) are causing an issue, it could be something you wouldn't imagine would cause a problem such as a blog extension or even an extension supplied as part of a theme.
Ultimately what you may have to do is take a full backup and re-install Magento, by which I mean creating a fresh magento directory on your web server by either downloading the package or using composer. then copying your env.php and media files and any locally created themes, to the fresh copy so that it uses your existing data and running the setup:upgrade to ensure that the database is up-to-date.
Then adding M2EPro. If that works OK, you can then start adding other extensions one by one until something breaks.
I have taken to having a complete duplicate set-up that I can test things on. I have been using Magento from the first release and have struggled with it ever since, despite being a technical person who has experience with managing web servers and some PHP development.
Note, one thing that will trip you up when making changes to Magento is that if you have php-opcache then you will need to restart Apache or php-fpm if you use Nginx to clear the php cache of pre-parsed php files.