I would bet that if your little seedlings were doing so well in that thin layer of mulch in the bed of your truck then they will thrive once in the actual ground. I know that your soil is mostly sand and pretty nutrient deficient, but wood chips are also pretty nutrient deficient. What makes wood chips great for gardening is the carbon makes for a home for microbes and other soil dwelling organisms. When in contact with the ground, woodchips will actually attract all sorts of soil denizens.
Those loquat trees by the way, are probably going to make great pioneer trees, sending roots nice and deep, dredging up other soil nutrients and adding their own leaf litter to the ground. Assuming that they don't grow out of control, they can be a great asset and resource for your land. I have Autumn Olive on my property, normally an invasive weed tree introduced in the '30s as a shelter belt tree because they grow VERY fast and are tolerant of most any weather condition--heat, cold, drought and flood etc. They have become something of a mess in my area. However, one of the guiding principles of permaculture is "the problem is the solution." Since I mow my grassy areas that otherwise would grow thick with Autumn Olive, mine are confined to a dense hedge/living fence. Every couple of years I cut mine back about 2' (out of more than 10') and chip them up for mulch, thus turning a weed into a resource. My point is that those trees you are planting, if growing out of control in the future or just need thinning, could make for a great source of wood for wood chips or huglekulture.
But in the meantime, I think you have yourself a great pioneer tree.