If you want to be sure, call the GfdS:
https://gfds.de/vornamen/gutachten-fuer-das-standesamt/#
You could also check the "Internationales Handbuch der Vornamen":
https://www.vfst.de/fachliteratur/produkte/internationales-handbuch-vornamenThis is, what
German registration offices use.
There shouldn't be any problem in accepting a unisex name. This is legal and since 2008 they are accepted without a gender-specific middle name.
It's only forbidden to use female name on a boy or vice versa (exception:
Maria as middle name for boys).
I actually couldn't find any source of Franzis as a name - male or female.
I found this link, but I guess, it won't help you: Franzis has been rejected as a male name in Iceland.
https://www.nordicnames.de/wiki/List_of_rejected_Icelandic_male_names
Why not name him
Francis? Or
Ffransis - which is pronounced similar to Franzis.