You only get one action, and up to one bonus action, each round. If you've already used your bonus action (but not your action - maybe you needed to cast Healing Word) then you would be unable to do this tumble thing if it required a bonus action.
yes, thanks, that makes sense of course - I'm still not entirely in the right 5E frame of mind...
You are not remembering the rules accurately. In Pathfinder, the DC to move through an enemy's space was 5 + CMD, so it scaled with pretty much everyone. To contrast, it doesn't necessarily scale in 5E, because Bounded Accuracy means your bonus doesn't increase with level unless it's something you're specifically good at.
correct, that was probably 3.5E... in any case I remember my monk weaving very easily through any battlefield control challenge... ;-)
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