Hi Karen
Indeed, welcome to bonsai!
Casuarinas can be ideal or a bit troublesome as bonsai. Most people, using the commonest grown (A littoralis, A cunninghamiana, A torulosa), find them very tolerant to bonsai horticulture (top pruning, root pruning). I've met a few people who have had troubles, and I can't determine exactly why.
I've grown a few A inophloia, because of the stunning bark. Horticulturally, they are fine, but getting that bark requires a bit more 'special' treatment, than one usually uses. For many years, I grew them in ordinary bonsai pots and the trunks stayed narrow, or increased very slowly in diameter. After giving the matter some thought, I worked out that to get those wooly, stringy bits of bark, the trunk needed to increase in diameter quite a lot, because they come from the splitting of the old bark - no diameter increase = no bark splitting. So I potted my tree into the largest bonsai pot I had, giving the tree lots of root space (and that is the key). The trunk fattened noticeably each subsequent season, and I'm now rewarded with the early evidence of wool production .

. It will take a few more years to be eye-catching, but I'm on the right track. I can't say if growing the ground would be successful too. I'm sure it would stimulate the wooly bark through good trunk growth, but I just don't know if they survive digging readily. Worth a try. I'd predict they would, but we need evidence.
I've also let the branches grow with only limited pruning at this stage, to help provide lots of energy to fuel the trunk growth. My experiences show that when cut back severely, that is back to bare branches/trunk, the remaining branches/ trunk sprout prolifically. So one follows the path of: develop the trunk first, selecting branches for the main structure as opportunity arises, but focussing first of trunk growth. You need lots of greenery to fuel the good woody growth, out of which comes the shaggy bark.
The bulb at the base of the trunk is a classic 'ligotuber'. Many Australian species have them. As your tree ages, and you allow the trunk to increase in diameter, the l-tuber will blend with the trunk, giving it an attractive swelling at the base. On my tree, the l-tuber has not produced the wooly bark, rather a finely bubbly surface, from which many new shoots sprout over summer. I just rub them off. The contrast between the wooly and bubbly parts, at this stage, just add to the great character of what this species offers. One shouldn't try to straighten bent noses - enjoy the personality behind!
I'll try to append some pics of mine, but I'm not always successful with this simple, (modern

) activity.
Cheers
Roger
[PS: it looks like I can only add one pic at a time

. I'll try a second post with another pic.]