that would be a convention or configuration debate
I prefer convention for systemd. The justification for have a separate timer file instead it being in the unit file would be making it easier for someone to quickly look at what services are running on timers vs running by targets.
if timers where in the services file you would have to open each one to find out what had a timer, with a separate file you just look at which service as a timer provided your naming is standardized)