You seem to have made some assumptions which are not quite supported with fact. The 16-24 helicopters which the LHD is expected to carry will be in support of the troops the LHD lands. Any F-35B carried aboard a Canberra-class is going to be at the space/displacement expense of one (or most likely more) of those helicopters. Secondly, there is a limited amount of space/displacement available aboard for aviation fuels, IIRC the figure of 800 tons available for aviation fuel comes to mind, any fuel for F-35B's will be at the expense of fuel for the embarked helicopters. Thirdly, there is also a limited amount of space/displacement in the armoury available for munitions to equip any aircraft carried aboard, any weapons for the F-35B will also be at the expense of munitions for the helicopters.
Yes, a Canberra-class would have sufficient space to carry more fuel and munitions aboard once the vehicles are disembarked, or if one or more M1 Abrams tanks are not transported... Any space and displacement 'gained' this way would not be configured to safely carry explosive or combustible materials safely. This would be a dangerous and stupid situation to find oneself in in a benign environment, since serious damage and potential loss of the ship could result from a simple and common accident. In a high threat environment, such an operating condition would IMO be virtually criminal except in extremis, when a capability was required and there was absolutely no other way to arrange it.
Now, it would be possible to have the HMAS Success attached to the task force to provide additional fuel bunkerage for the F-35B's embarked, as well as a stockpile of munitions to arm them with... IMO such a solution is not desirable or really practical. For one thing, given the estimates used above about F-35B sortie rates and fuel consumption, the onboard aviation fuel stocks would be depleted in just under a week, and that is assuming that all the fuel carried and used was just for the F-35B's. Now, the Canberra-class LHD is currently expected to have a 50 day endurance before requiring replenishment, not a 7 day endurance, which I expect would seriously cut into operations. Not to mention I would expect that RAS would more likely need to be conducted every 4-5 days, given that likely no more than two-thirds of the aviation fuel aboard would be available for the F-35B's.
And this whole situation still ignores just how useful 6 embarked F-35B's would be to an amphibious task force. Just 6 F-35B's as discussed previously is really insufficient to maintain a CAP around the task force, given operational taskings and the minor little detail like how often the LHD would need to RAS and possibly preclude flight ops while doing so. If the intent is just to provide a strike or CAS capability, then six F-35B's is either overkill or significantly insufficient. In terms of being overkill, if the objective is just to have an airborne platform from which to fire a cannon, launch missiles or drop bombs with little or no opposing enemy air defence, then a LO fighter platform is not needed. In fact, the Tiger ARH might be a better alternative, and there would likely not be a large and hardened target which the ARH could not engage. OTOH, six F-35B's could also be insufficient if there was an IADS, and/or a significant number of protected and hardened targets, since even with the F-35B being a LO platform, a maximum six aircraft surge flight can only strike so many targets at once, and that small a number would not be in a position to sanitize an area quickly unless there was already little need to do so.
-Cheers