The sollution you got is probably what you should do.
But It got me a bit curious, can you not use systemd for this?
So I searched the interweb and bumped into this.
If you feel curious and adventurous, maybe starting transmission with systemd-inhibit transmission from cli would prevent sleep while transmission is running.
I did not know about this before, and have never tested anything with it, but made me curious, maybe you too. 
systemd-inhibit may be used to execute a program with a shutdown, sleep, or idle inhibitor lock taken. The lock will be acquired before the specified command line is executed and released afterwards.
systemd-inhibit