Regardless of the age, it takes foliage to maintain a healthy tree.
I don’t know much about niwaki per se. So I googled it, then looked at pictures. All the trees had significantly more foliage on them than this one.
I really don’t see there’s a difference, horticulturally, between a niwaki in a tub and a bonsai tree in a pot. I did see that your tree appears to be in “garden soil” or pitting soil, which I know is a poor choice for JBP. They prefer a much better draining soil mix than what potting soil provides.
Yes, I know foliage is needed for health.
But, most of the energy collected is from previous Fall and Summer (when sap is running the most).
Also, I think Momiage on niwaki is so much more bare than a bonsai... because who wants to do it multiple times per year to let's say a 15-25' tree? (in the cold of winter, on a ladder and in the outdoors). That's my guess anyways. I'm sure it's easier for a much smaller bonsai.
Of course, if you just google "niwaki" it'll show a bunch of trees with a lot of foliage. Mine has a lot of foliage too in the growing season, lol.
What you Won't find many pics of, is a JBP niwaki right after a fresh Momiage needle-pluck. So, I attached the few pics I can find... not my pics.. but those are in Japan. I've seen it much more bare than that (like mine) everywhere in Japan and also at niwaki nurseries in SoCal.
You don't see a difference, horticulturally? Maybe a niwaki is a lot bigger and has much more foliage in the growing season than an average or small bonsai. And the branches (as Shibui mentioned) are much more spread apart. So, perhaps, with this greater branch distance and spread, requires more balancing and more needle reduction than a smaller bonsai.
The soil is 40-50% pumice (and some scoria) and the rest is typical organic matter (pine and fir bark, "potting mix," etc.). Still good drainage with all that pumice, and also the needed organics (fines and all) to survive SoCal climate.
It's not a poor choice... it's the best choice for super dry (and hot) SoCal where the humidity goes to single-digit % often with the occasional Santa Ana winds. Even for small bonsai, it is a good mix for SoCal (50% inorganic).
I got my soil mixture recommendation from the JBP grower posted above (the eBay grower... not the niwaki nursery... but the niwaki nursery and House of Bonsai in LA uses a similar mix for their niwaki of bark and pumice)... the eBay grower is originally from LA and knows all the old-school/well-known JBP growers in SoCal/US, so I think he'd know my climate.
100% inorganic/aggregate mixes are for people that water their bonsai 2-3x per day in SoCal during the summer or have a nursery with workers or have timed irrigation.