If the files in the transmission download dir are not accessible by other services (radarr, sonarr, etc), the permissions of the download dir should be changed to enforce files created will be set to the sc-download group rather than root. This can be done by setting the setgid bit after changing the group of the folder:

sudo chgrp "sc-download" /path/to/transmission/download/dir
sudo chmod g+s /path/to/transmission/download/dir

Configuring Transmission to use a specific umask:
https://askubuntu.com/questions/157455/how-to-change-the-default-permissions-of-files-created-by-transmission-daemon

sudo synopkgctl stop transmission

sudo vi /volume1/\@appstore/transmission/var/settings.json #DSM6
sudo vi /volume1/\@appdata/transmission/settings.json #DSM7
# change umask to desired value 775/664 = 2, 755/644=18 
#echo $(( 8#002 ))
#2
#echo $(( 8#022 ))
#18
#

sudo synopkgctl start transmission

DSM7 Directories:

/volume1/\@appconf/transmission/  - empty
/volume1/\@appdata/transmission/  - conf+log
/volume1/\@apphome/transmission/  - empty
/volume1/\@appstore/transmission/ - app files
/volume1/\@apptemp/transmission/  - empty