I don't think it is unusual to have zero or almost zero districts in the Ancient Era unless trying to get a religion. In the Ancient Era I generally focus on empire expansion and military conquest. I'll place districts to lock in the cost if it makes sense, but mostly not spend time finishing them until the Classic Era.
In my current game I reached a Free Inquiry Golden Age in the Classic Era, but had zero commercial/harbor districts for about 18 turns to take advantage of it - still got the eureka bonuses.
Placement can get an 'easy' +8 historic bonus points if you settle a city on/near a river and just one nearby mountain. If you can place a campus next to the mountain, city, and Government Plaza, you'll get +3 adjacency for splendid historic point bonus (note: must complete the government plaza first). For the Commercial District, next to river, city, and gov plaza gets the +4 adjacency and the splendid historical point bonus. Of course, that requires a size 7 city, but I get there by harvesting food.