Personally, I would begin with breath meditation each day, switching to Metta or Asubha if my mind seemed a little too uneasy to focus on the breath (or sometimes I just hankered for a change of pace
If my mind is uneasy I pay attention to the uneasiness, then back to the breath, then back to the uneasiness, ... and so on. I don't really pursue Metta or Abusha at all... although I'll (of course!) attend to them if they happen to come up during the uneasiness. If I try forcing another technique then I get thoughts like, "do I really have to do the body scan?", and feelings of frustration and boredom. So I don't force it
Can meditating on the breath take you all the way? Of course other things will crop up automatically (thoughts, feelings, aches, spaciousness...) and attention will move to them.
Are watching the breath & paying attention to all experiences the only things you *need* to force yourself to do on the "meditation" part of the path? (Assuming you really like breath meditation and don't like other techniques...)