Since mastery xp gain is linear the best early game strategy is to die quick, maybe make the xp gain exponential so surviving longer is more useful early game so a formula like x*1.5^x
I kinda fixed this problem later in the game, where you can get a lot of multipliers to mastery xp gain, so longer runs are much more worthwhile than the shorter ones, but I'm aware there is a gap, where short runs are more efficient. Maybe I swap store and tavern unlocks, so you would get access to some mastery gain multipliers sooner. And this could also help with the other problem you commented on before.