There are tons more techniques for escaping mount, but the ones you have been taught are the ones you should focus on. There are several reasons for this.
Focus on fundamentals
The two escapes you know are arguably the most straightforward and efficient methods of escaping mount. In addition, they both develop absolutely critical grappling movement skills: the bridge and the shrimp. These two movements will be used in all sorts of situations. They are also complementary: if the person on top stops one, the other is often easier to apply. Instead of looking for other techniques, you'd do well to put your effort into applying these two techniques against a variety of opponents.
The other major reason to stick with these is that they are high percentage techniques. Even the best jiujitsieros have trouble with these two escapes. They are used in elite BJJ competition. Don't discount them.
Don't be a YouTube grappler
Aesopian, a BJJ black belt who has been prolific on the web since he was a blue belt, used to be a voracious reader/watcher of instructional tapes and books. He has this to say about learning in this manner:
You find some interesting techniques in a book or online. You glance them over and make a note to try later. At class, you’ve got your attention split between what is being taught in front of you and the half-forgotten tutorials floating around in your head. When sparring comes, you drop whatever the day’s lesson was and fumble to piece together something else entirely.
He goes on at length. This is the fate of the grappler who attempts to learn technique from the internet or instructionals, instead of in class. Don't learn your technique from YouTube. Learn and drill in class.
(That being said, learning techniques from video can be marginally useful in some limited sense.)
Improving your upa
The bridge is a fundamental movement not just to escaping mount, and not just to Brazilian jiu-jitsu, but to all of BJJ, all grappling, and all fighting. Getting your bridge to be technically efficient as well as powerful is a great goal no matter what level of jiu-jitsu you have.
When I'm working on my upa (bridge), I work on strength, power, and technique in the following ways:
- (Technique) Movement drilling - I practice the movement on my own. I make sure I get my feet close to my butt, I make sure I'm not exposing my arm, I make sure I'm extending my hips fully, I make sure I'm pivoting on my shoulder. There are a variety of movements
- (Strength) Barbell hip thrusts - I hope the implications of being ferociously strong in this movement do not need to be explained. Other general strength exercises that assist this movement are the barbell squat and deadlift.
- (Power) Olympic lifts - The barbell power clean is a tremendously successful method of translating strength gained from other strength work into speed-strength (power), which is the most relevant attribute to athletics.
- (Technique) Positional drilling - The simplest way to get gobs of training time in mount escapes is to drill it repeatedly. Give your partner mount in free rolling, or outside of class get a skilled partner on top of you. Set the timer for ten minutes and switch every time you escape. Have them resist, and try to submit you, to the best of their ability. Do this with as many partners as possible, and you will get very familiar with the minutiae of what works for escaping.