"I could also make a template to cut a mortise into the sides"
If you decide to use loose tenons (I'd), you can refer to the manual and do it as a Carcase Butt Joint. Pay attention to this tip, which is applicable to your case:
[attachimg=1]
There's no need to make any template unless a router rather than the DF500 is intended to be used for this project.
For structural strength and rigidity, the seat and bottom are also joined to the back with dominoes (do plan for wood movement).
P.S. Cut all the joints before shaping the sides.
This diagram would give you a general idea of the suggested domino placements:
[attachimg=2]
I'd also treat the joinery for the dividers and the seat/bottom as a carcase butt joint, and use the same procedures as laid out in the manual to mill the mortises (but this time, the second tip may apply, if it's 20mm thick or less, or roughly center the mortises). Needless to say, I'd also use loose tenons for all the drawers.