I've been betting on UFC fights for quite a few years now, and trust me, I've tried my fair share of platforms. Your requirements are pretty close to what I look for, and BetUS Sportsbook stands above the rest in my opinion.
They offer a lot of betting options, not just who will win but also more intricate details like the method of victory (KO, submission, etc.).
They have live bets and their mobile app is a life saver, to place bets in front of the TV from the couch. And they have a welcome bonus of $3000 if you deposit at least $50.
A great alternative for them is Everygame Sportsbook. They list a lot of UFC matches, although I'm not certain about their live betting options. Unfortunately, they don't have a mobile app, but their site is "okay" in mobile too. Their welcome bonus is a bit lower at $750, but it's still decent.