"Basically i would have to create some gigantic SQL-statements to get all the custom form informations and so on from each ticket."
Yes.... I don't know that someone ever created or built an SQL statement or script to export tickets.
So if you really need and want to do it, I would suggest to may get in contact with the developers to do it as a plugin or in a way that it can be integrated into the core of osTicket.
But maybe @[deleted] knows if someone else did already wrote a script or another solution to export tickets.
Cheers,
Michael
PS: Don't know if it's a typo or not, but in English the plural of information is information and not informations ;)