What I notice is that it is not raised very much above grade. So that if there is some reason it needs to have better drainage than what is prevailing on the flat there the bed will have to be a lot deeper. Also the area of dark soil is not very wide - like various other Dissectum maples 'Tamuke yama' is not actually a dwarf bush, with 13 ft having been stated as a typical mature height. So if you are trying to plan for the long term at all your new spreading and mounding habit tree will probably need a suitable soil area of at least comparable width to grow in over time*.

And if you have added a lot of organic matter this is going to decompose and disappear, resulting in significant settling. You should instead use a good topsoil with a high mineral content, so that there is not a lot of shrinkage later.

*Of course, it might sometimes take a weeping lace-leaf maple 70-100 years to reach full height! But if yours thrives perhaps it will become several feet tall and wide in some proportionate fraction of that amount of time - commercial growers in the Willamette Valley (USDA 8) are certainly able to come up with Dissectum maple specimens that are pretty big within reasonable periods, for prices the market will bear

Your tree will be fine there. Growers can produce full sized trees (referring to most weeping dissectums) in a 24" wooden box! And the ultimate height of the tree will depend on where the graft is located, not on some arbitrary height listing. But it can get quite wide.

A nursery specimen in a 24 inch box is not going to stay in it for the rest of its life. Grafting only adds distance between point of grafting and ground. Most lace-leaf maples are not grafted several feet in the air - when one is a fully developed specimen the low height at which it was first grafted will have a trivial effect on how tall it ended up getting.

"when one is a fully developed specimen the low height at which it was first grafted will have a trivial effect on how tall it ended up getting."

I will disagree with that contention :-) Location of the graft union has a major bearing on how tall the plant will be at maturity, provided it is not staked or trained to grow more vertically. The natural growth habit of a weeping dissectum like Tamukeyama is a close layering of the branches with minimal vertical extension. That's why those with very low grafts (most common) form low, dense, mushroom shaped plants that will never get anywhere close to 13' tall!!