Good to know about smoother causing desyncs, my brother and I play with it and I like being able to play omniscient to get the 25% production malus, but we do desync, I mean only once or twice every few games, but..
I think smoother still works, I think it works well for crowded maps, which the AI settles very quickly. I go back and forth between using Real Strategy or not. I think it leads to a similar unit buildup as what is described for AI booster, which is a must for enjoyable warfare in mid-late game.
I also like to give the AI a 25 point boost to combat strength against districts (I tweak radiant stronger cities for this) and triple pseudoyield_city_base in Real Strategy, which seems to affect war declaration and the desire to actually pick a target and attack. Then when the AI does attack, they can capture an unwalled city in 2 hits and a walled city in 3-5. (By default radiant strengthens cities, so you have to revert this, and increase the strength bonus.) It also helps them capture cities off each other, and the top half of AI tend to capture 5-6 cities per game.
With these settings I lose a city mid-game almost every time I play (most recently, Norway took advantage of me massing my navy against a neighbor on the other side of my empire), and recently I have been losing cities/games due to early warfare most often (at least on Diety).
For challenge, I think you want the AI a half-to-full era ahead of you in tech. Also helps to use shuffle tech tree or something that eliminates the tech bee lines, so that the player can’t simply keep up militarily at the expense of infrastructure. Smoother difficulty gives a pretty meaningful boost to science and culture that helps with this.
I’m curious to hear if folks see the AI using their larger armies meaningful, often they send too little to the front line and attack in manageable waves, and their larger force is more effective for defense. I have fiddled with this, but this actually might be semi-optimal for them, since otherwise they can kill off each other’s forces and leave themselves vulnerable to the player. I haven’t as much lately, but I think a retooled dramatic ages is key to letting one AI snowball, since they gain ground without losing as many units. And even just with the above changes, I’m seeing a few empires killed off as late as the industrial era.