I managed to solve the issue I had with sideloadly not working. the ipad is now jailbroken, some interesting facts I found:

• system partition on 16 GB ipad, is 3GB
• on 9.3.6 you only have 100 MB empty space
• there are attempts to offload files and programs to /var but it is dangerous since a reboot could brick a tethered device
• the stashing app for this purpose is not compatible with 9.3.6
• ios is based on darwinOS so it basically is bsd.

my plans for the future:

• try to find the sourcecode for the stashing app so I can port it
• try and find an untether for 9.3.6 or make one myself
• try to run postmarketos, maybe also changing the partitions

possible issues:

• the bootloader is closed afaik so the device might brick if rebooted