kext custom loading enabled, app prevents usage

[realdadfish]

Instead of disabling SIP altogether and opening the security nightmare macOS already is even more, I decided to just enable custom-signed kernel extensions. After reboot I found however that the app doesn't recognize that and still tells me to disable SIP completely, even though the kernel module could apparently load:

➜  ~ sudo "/Applications/Shrink Trackpad.app/Contents/MacOS/app_shrinkTrackpad"
Kext with invalid signatured (-67050) allowed: <OSKext 0x7fc3a4602750 [0x7fff8b60daf0]> { URL = "file:///Applications/Shrink%20Trackpad.app/Contents/Resources/kext_shrink_trackpad.kext/", ID = "com.cocoahuke.kext-shrink-trackpad" }
Code Signing Failure: code signature is invalid
Invalid signature -67050 for kext <OSKext 0x7fc3a4602750 [0x7fff8b60daf0]> { URL = "file:///Applications/Shrink%20Trackpad.app/Contents/Resources/kext_shrink_trackpad.kext/", ID = "com.cocoahuke.kext-shrink-trackpad" }
/System/Library/LaunchDaemons/com.cocoahuke.shrinkTrackpad.plist: No such file or directory
2018-10-25 12:14:25.229 app_shrinkTrackpad[1530:14077] Could not connect action, target class ViewController does not respond to -label_leftUpper:
*** Recognized (0x69) family*** (30 cols X 22 rows)
^C
➜  ~ csrutil status
System Integrity Protection status: enabled (Custom Configuration).

Configuration:
	Apple Internal: disabled
	Kext Signing: disabled
	Filesystem Protections: enabled
	Debugging Restrictions: enabled
	DTrace Restrictions: enabled
	NVRAM Protections: enabled
	BaseSystem Verification: enabled

This is an unsupported configuration, likely to break in the future and leave your machine in an unknown state.

What can I do here (except really disabling SIP completely)?