Get that thumb back around to the center of the neck or below, keep it parallel with the second finger. space one per fret starting at the fifth fret and go up each string 1 2 3 4 , 1 2 3 4 1 2 3 4, 1 2 3 4, then go up a fret and repeat. go all the way up til you run out of frets then play 1 4 2 3 , 1 4 2 3, 1 4 23, 1 4 2 3, and so on, build up strength over time, then drop to 4th fret and do it,. any combination helps. 2341, like that. Whenever i spend a few weeks without playing I do those exercises to wake up my hands. It hurts, that's how you know it's working.
Permutations. They work for me too. Been doing them for years.