The best part to collect wouldn't be bark at all, IMO. This is because Acacias tend to be sensitive plants, and bark damage often leads to sickness or death in them.
If the leaves can be used (and I'm not sure if maidenii leaves can) then I would use the leaves, even if the bark is more potent. But don't strip
too many of them from one plant, as that can be just as bad.
I think the bark on the outermost (small) branches can be used with minimal damage to the tree, but I'd wait for someone else to chime in before I went out and started stripping any kind of bark.
Respect the plants and they'll respect you back

07:45:13 ‹Bonné›The least interesting part of a psychedelic experience is definitely the visuals.