Just mount?
I suggest the knee push escape because I think it is the highest percentage.
And at its most basic for self defence. Cover your head. Buck your hips up, knee them in the bum. Turn on your side while still covering and try to touch your elbows to their knee.
Then escape.
You are relying on them not being able to hold you down while hitting you and not being able to hit you while holding you down.
I don't fart around with trying to hold them down from there. I just mash that escape. (That back hug Tony mentioned.)
So like this but just trying to be more compact.