What are some of the strongest versatile civs right now?
Versatile as in policy/playstyle choice wise? So you can do well with more than one playstyle?
Poland is slightly pushed towards wide with progress/authority + fealty, but can generally do whatever it pleases. No big push towards anything, though Ducal Stable, Fealty and lots of pastures are an obvious synergy.
Maya can go all trees and are strong. Tradition/Progress are the best, but no pressure.
Byzantium the same, you can choose whatever religion you want and have tons of faith. Can go GP focused beliefs/choices, can go warmongering, can be peaceful. You however will have to pick beliefs that fit each other.
Ethiopia also works for all three starting trees though Tradition the least imho, but in medieval Fealty's probably where you will wish to go.
China too, go tall and spam GWs, or conquer everything. Raze, resettle with your own, bonus yields. Everything works. Good for beginners. Just don't tech to the next era too fast, it might not always be worth it
Inca are sort of versatile too, but a part of it disappears without RNG. If you start with no mountains and only several hills you still benefit, but that's nothing compared to what you could be getting otherwise. The most powerful with the right RNG, the only flaw is Culture and that can be indirectly gotten with bonus Production/Food from the UI.
This applies to versions before the last two, from my experience expansion is not viable without adjustments now.
Mongolia’s the strongest right now. The annex on heavy tribute is overtweaked at the moment, and extremely dangerous. Versatile, not so much.
Maybe they'd be so in a version where not everyone is emo, happiness exists and not everything is despair, but right now I think getting a city like that is negative, especially early on where you'll have that useless puppeted City State you cannot raze cost you 2-3 happiness, 7 Gold while giving you 0.4-0.8


before medieval, which is rounded down to nothing because only full numbers count.
With Mongolia is it worth dipping into Tradition for an Authority border blobs strategy or does that not really work anymore?
This tactic even as Russia is pretty bad in my opinion. You lose yields per city from Authority while gaining situational yields in the capital that scale far worse with bigger city count, lose having your yields per border growth doubled from Authority finisher, lose a few Tribute yields per border growth as the borders grew a few times from you picking Tradition > 1st policy into Authority, lose having your scaled per era border yields later, your UA becomes less powerful in the long run and you have problems building stuff because less production. You slightly snowball technologically at first, only to have that very ball disintegrate itself from your greed.
It was always overrated and even before Authority got Tribute double yields in the finisher it wasn't good.