Enable mod_http2, I think that's it, but I never tried.
Real domain names would help here. Besides it makes no sense to point ns1/ns2 to the same machine, same for ns5/ns6.
Which extension did you install? Maybe it's only that extension that is broken...
In Firefox you can bypass certificate errors, log into your ISPConfig and recreate the LE certificate
Which Joomla! version are you using? How do you upload the setup file to Joomla!? Which php mode are you using?
Keep in mind that default values of server and actual values of database/table might be different. Which ones did you check in phpmyadmin?
Can you telnet to the port 465 on the system? Maybe a firewall is blocking the access...
The files need to have the permissions of the web. You should upload them using a web ftp user or chroot user or chmod them afterwards.
/dev/null is more like a file.
What's the output of netstat -taupn | grep master
Did you reconfigure services?
devnull before colon is a virtual user which gets redirected to /dev/null
Postfix rejects all but known email addresses, but yours is known, or you will not be able to send mail using it.
If you created and activated the alias in steps 1) and 2), then yes. The mails arriving at that address will get discarded.
Separate names with a comma.