Cutting/tipping the shoots doesn’t push more growth on the spurs, it causes the flowers to set fruit.
That is every spring for us 
I think Sihong is a good choice for a 3rd tree, with fruit which is pretty different from the other 2. And it is supposed to be much more productive in warm/sunny areas than where I am. I got my Sihongs several years ago from Englands Orchard in KY. You’d need to email/call him to see if he will have any for next spring.
If you want a good 3rd option that you can likely find a tree of, I’d suggest Sugar Cane. A potted one is on sale at Just Fruits and Exotics, or you could wait for next winter and likely get a bare root one from Grow Organic.
I have one R4T3 tree and have been grafting over it. The fruit has been OK, but not great. If you want to see my complete writeup:
If you want interesting shapes, there are several more radical options:
Mushroom:
Hulu/Gourd/Mopan/Millstone:
Teapot:
I’ve grafted Gourd and Teapot, but haven’t sampled the fruit yet. In general, the strange-shaped fruit is not though to be as high-quality as the more mundane looking fruit.
But, you may be able to buy a Mushroom tree from Chinese Red Date.