I don't want to use the default settings folder at

C:\Users\<username>\AppData\Roaming\InkScape\

but use my own here

D:\tools\Inkscape\mysettings\

for ALL related preferences files (except the installation itself).

As far as I understand I have to write

D:\tools\Inkscape\mysettings\

into Settings--->System--->Shared Default resources folder.

Is this sufficient?

or does "shared folder" mean sharing between all users on current computer?

Where does Inkscape remember this new user settings folder?
In the new user settings folder this is not possible.

Its a chicken-egg problem:

Inkscape must read the (unknown) user settings folder to get the user settings folder location.

So there must be a reference outside the user settings folder

I'd ask here: https://chat.inkscape.org/channel/inkscape_user