I did it! (and it seems to work)
So after I had no reply at all (on all sites I've posted this question) I went ahead and did what I planned.
I've logged in my NAS via ssh as admin.
Next step was to modify the follwing two files so that all users created by me have a UID and GID starting with 1000
[~] # cat /etc/passwd
admin:x:0:0:administrators:/share/homes/admin:/bin/sh
guest:x:65534:65534:guest:/share/homes/guest:/bin/sh
httpdusr:x:99:100:Apache httpd user:/tmp:/bin/sh
mainuser:x:500:100:Linux User,,,:/share/homes/mainuser:/bin/sh
test-consumer1:x:503:100:Linux User,,,:/share/homes/test-consumer1:/bin/sh
[~] # cat /etc/group
administrators:x:0:admin
everyone:x:100:admin
chagned into
[~] # cat /etc/passwd
admin:x:0:0:administrators:/share/homes/admin:/bin/sh
guest:x:65534:65534:guest:/share/homes/guest:/bin/sh
httpdusr:x:99:100:Apache httpd user:/tmp:/bin/sh
mainuser:x:1000:1000:Linux User,,,:/share/homes/mainuser:/bin/sh
test-consumer1:x:1003:1000:Linux User,,,:/share/homes/test-consumer1:/bin/sh
[~] # cat /etc/group
administrators:x:0:admin
everyone:x:1000:admin
next step was to chown and chgrp all files on the NAS. As the find that was available would not perform the exec command I did go through the dirs myself
lrwxrwxrwx 1 admin administ 18 May 19 14:16 Qdownload -> HDA_DATA/Qdownload/
lrwxrwxrwx 1 admin administ 20 May 19 14:16 Qmultimedia -> HDA_DATA/Qmultimedia/
lrwxrwxrwx 1 admin administ 20 May 19 14:16 Qrecordings -> HDA_DATA/Qrecordings/
lrwxrwxrwx 1 admin administ 13 May 19 14:16 Qusb -> HDA_DATA/Qusb/
lrwxrwxrwx 1 admin administ 13 May 19 14:16 Qweb -> HDA_DATA/Qweb/
lrwxrwxrwx 1 admin administ 10 May 19 14:16 b -> HDA_DATA/b/
lrwxrwxrwx 1 admin administ 15 May 19 14:16 backup -> HDA_DATA/backup/
lrwxrwxrwx 1 admin administ 13 May 19 14:16 home -> HDA_DATA/home/
lrwxrwxrwx 1 admin administ 10 May 19 14:16 m -> HDA_DATA/m/
lrwxrwxrwx 1 admin administ 10 May 19 14:16 v -> HDA_DATA/v/
[/share/HDA_DATA/] # chown -R 1000:1000 ./*
I could fix all files and folder at once as there was only one main user that had uploaded files to the NAS.
Next I did restart my NAS and checked logging in as the users that I just changed.
I know this might not me the most elegant, fastest or right way of doing this but it worked for me and I hope this helps someone else.
Check here as well.