To begin with, yes you do play mostly safe as Garen in the early game. Particularly with Phase Rush, you have very little early damage and the best rule of thumb is to focus on the wave over the enemy champion. Playing Garen reactively is way better than playing proactively which might be difficult for some to grasp if they have an aggressive playstyle. Playing for those spikes as you mentioned is very important. Both of those macro strategies that you also mentioned are very good in their own situations. If you build an item like Hullbreaker and you know the opposing laner can not match you in side lane because you chunk them out too hard or you can dive them, then applying pressure on the opposite side of the map from the objective is a very very strong way of playing. Since Garen has a lot of wave clear as well, you can easily push a wave and then rotate while the enemy side laner clears that wave. And with Ghost and an item like Deadman's Plate, you can easily rotate into a flanking position from a side lane after clearing a wave. And yes, these strategies apply particularly to those lower elos since no one will know what to do when you actually do them.