I am playing for the long-term, and don’t care about ranking or how fast I progress, so very much a causal playing style. Given that, I find my lower age units are great for GE and quests. In each era, I always start GE with lowest age units I have. That wins a large number of current age units, which I keep for GBG. Since I’m going for higher era units, I save my techs and only get current age units through GE and GBG.
Currently in Industrial, and complete all 64 each week. Camping in EMA to get core fighting GBs to a decent level was well worth it. Now, typically finish GE1 with Iron age units, and halfway through GE 2 switch to EMA. (I'll run out of IrA units soon, so then will start with EMA units, then HMA, etc.).
For GE 3 I use LMA-HMA units, and for GE 4 colonial ones. Auto-fight GE 1 and 2, part of 3, and manually fight most of the rest. Lose very, very few battles with this approach.