You might find my video series on short-stacked play useful. It doesn’t deal with stack sizes too much beyond 20bb in any great depth, but it introduces the kind of concerns that you need to bear in mind when you’re a maximum of four bets deep preflop.

It’s also worth noting that 20bb and 30bb are wildly different in terms of the options you have available to you preflop in the majority of situations. I wouldn’t recommend trying to study the two sizes in the same way. I’d recommend doing some studying of 20bb play, then comparing that to deeper-stacked strategy and trying to extrapolate how play changes as you get closer to 20bb on a sliding scale – there’s no unilateral adjustment you can make once you get below a certain level that will make things easier. It all depends on the situation.