Well Ethiopians is maybe not the best example because against many civs you will struggle in Imperial Age on Arabia.
So the general question should rather be: How will the lategame composition look like and do I want to let the game get there?
So many games with Ethiopians 3 TC will not be the play and you will be better off pushing anyways.
Generally speaking, being behind in the boom is not a good thing.
It’s like when you attempt an FC, fail and ending up in “something between” which is never a good play.
You should make the right amount of military, try to get map control, find holes, do damage, retreat and build towards a good composition.
What the right amount of military is?
It depends.