Domination help?

So I'm playing on Emperor and have had a good few pretty comfortable culture, science, and diplomatic wins. But domination still eludes me. Maintaining happiness and a positive GPT in the early game when going domination is extremely hard for me, and when I focus on military my science, which I usually focus on to catch up with the AI, really suffers and I get massively behind. It seems to ruin the whole point of going for a domination victory to raze every city you capture other than capitals, but is this necessary? And how can I maintain a decent science output on the higher difficulty levels to stay with the AI whilst also having a threatening military?

Play as Mongols. Have 2-3 cities. Get national college. Go down Liberty tree, get great scientist, use to bulb whatever tech you're on. Sell extra luxes for GPT to build up treasury. Build 5-6 horsemen. Beeline Civil Service. Upgrade all but two horsemen to Keshiks. Raze every city other than capitols unless there are gold producing luxuries. Build new Keshiks as you can to add to your forces. Leave archers behind in conquered capitols to control barbs.

That's the short version. Works at least up to King difficulty... haven't tried it past that yet.

I run into the same problem in BNW. Generally yes I raise the city unless it has a luxury I don't already have. Or, if you really want the city for a strategic reason, you could raze it, then it will lose a population each turn and at some point stop the rasing, that way it will have less pop and therefore less hapiness. I also tend to make sure to produce all the happiness buildings, and if you ally a mercantile civ, that is usually good for 10 hapiness or so.
Also, I used to always puppet my cities in part because I am lazy and don't want to pick everything for it to build, and the other part being I like policies quicker. For puppeted cities, it seems the CPU makes some bad choice of buildings to build,and I tend to be able to make the happiness buildings that way that got neglected.

burn baby burn! raze cities and also, I find it helps to skip cities. cripple a civ, take the capital, and sign a peace treaty when they are ZERO threat. then move on to the next one. This worked for me in the one domination game I played, I was leaving civs with a handful of cities, but that was fine. Although, my science still got bogged down and I had to move quickly to finish.