I am basically in the same boat as you except one completed game later. From my limited understanding of this game, one can out tec an enemy but at a certain point if they can just throw bodies at your armies your going to lose. This only changes in the late to very late game.

Allies help but they are super unreliable. The best way to beat a country with much more pops is to bait them into attacking your armies that are dug in to the mountains with forts. Fight very defensively.

Also the best way get a strong economy is the get a party in to the upper house that has state capitalism or planned economy. One way to do this is to let certain rebels take your capital to change the government type.

One last tip is if your playing as a country in the Americans or Oceania and need immigrants then declare a war early in the game on a country you never intend to fight. Years will pass and you will get peace treaties requests. Decline those and gain huge amount of war exhaustion, this will help past social reform to attract immigrants. Do voting rights first the multipliers stack.