Nice choice with putting lava rock at the bottom instead.
For catts, it is probably unnecessary to add bits and pieces of sphagnum. But if it works for you, and continues to work nicely - then no problem with that MM. Just sometimes - if regular (not-adapted) catt roots stay wet due to sphagnum pieces keeping them wet, then those particular roots can go gray/grey mushy etc ----- running out of O2. But if no problem ----- then ok.
As for the rhizome level ...... I think you have got it absolutely right there.
Such as shown in the potting of this orchid at 4:40
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