That's a really nice space. Since this is the centerpiece of your garden, getting the right tree (and cultivar) is critical. JBP is an excellent choice, but I think the standard variety is a better choice than "Thunderhead," not only for bonsai but also for niwaki. I had a landscape Thunderhead, and although I didn't have it long enough to see its potential (I moved), even in a short period of time I was able to see its weaknesses as compared to a regular JBP. Here are my observations, both from my personal experience and from what I have read from others in this forum.
JBP
+ Vigorous grower
+ Barks up more quickly
+ Shorter needles
+ More refined branches
- Can grow too large to do niwaki if you ignore it for too long ("too long" probably means 5-10+ years of just letting it grow without any pruning; so, this probably isn't a problem)
JBP Thunderhead
+ Vigorous grower
+ Only 15' mature height, which means you can let it get away from you and not have to worry about it turning into too large a tree to do niwaki; however, if left alone...(see next item)
- Develops whorls extremely quickly, which means you have to stay on top of pruning to prevent bulging
- Thicker, more juvenile-looking branches
- Grafted?
While I think regular JBP is the better choice, admittedly, there is so much less info on Thunderhead, especially experiences from people doing niwaki on it over a long period of time. I can only find a few images of mature Thunderhead online...
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...and while it's possible the Thunderhead could look awesome as niwaki in the right hands, there aren't many examples. On the other hand, the regular JBP is proven as niwaki:
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Not sure on pricing, but I wonder if a place link this (
https://www.hanselmanlandscape.com/nursery/ornamental-pines/) has younger ornamental trees that would be under $1K including shipping (or, you could take a road trip to PA to pick it up!). Of course, there probably are closer options, especially in OC/LA, as
@LuZiKui said. Ultimately, you're going to need to purchase a pre-niwaki, and develop it yourself over the years.