This seems a good question. It is a source of problems for me that one is not allowed to refer to the thing-in-itself as a phenomenon, entity or thing.
A phenomenon depends on a noumenon (that has phenomenal properties) so to call the noumenon a phenomenon is confusing. In Buddhism's Abhidhamma pitaka (basket of teachings), which itemises the components of existence, Nibbana is called a phenomenon but uniquely among phenomena is without a noumenon. This is the approach I use, calling the final phenomenon a phenomenon but noting that it is different from all others. Otherwise a language problem arises and no word will do for the thing-in-itself (For instance, it is called a thing but defined as not a thing).
Note that Kant is not expert in this area but speculates. He speaks of multiple things-in-themselves which mightily confuses the issues. The idea that what lies behind phenomena is a large number of things-in-themselves is unworkable since by definition there is no way to tell two things-in-themselves apart. If we say there is just one ultimate phenomenon/noumenon then your question becomes a little more simple to address.
Kant supposes that we cannot know the thing-in-itself so must speculate on these issues but this view is not forced on us. For the mystic the thing-in-itself would be the 'I Am' of the Old Testament and this would knowable by identity. But this line of thought would take us OT.
You might like to check out the 'problem of attributes' which is immediately relevant. Here is Colin McGinn as a teenager wrestling with it.
“…[P]icture me sitting on a bench staring at a British mailbox on a blustery spring day in Blackpool. I had just been reading about the questions of substance and qualities, and was suitably transfixed. Is an object the sum of its qualities or does it have an existence that is some way goes beyond its qualities? The mailbox had a variety of qualities - it was red, cylindrical, metal, etc. - but it seemed to be more than just the collection of these; it was a thing, a “substance,” that had these qualities. But what was this substance that had those qualities? Did it lie behind them in some way, supporting them like the foundation of a house? If so, what was this underlying thing like - what qualities did it have? If it had some qualities, wouldn’t there be the same problem again, since it would also have to be distinct from these qualities? But if it had no qualities, what kind of thing could it be? How could these be something that had no qualities? So maybe we should say that there is nothing more to a mailbox than the qualities it manifests. And yet how can an object be just a set of abstract qualities? Isn’t it more solid and concrete than that? … I had a vague mental image of a grey amorphous something that constituted the underlying mailbox, to which its various manifest qualities mysteriously were attached… Yet as soon as I replaced this fuzzy image with the qualities by themselves, trying to think of the mailbox as just a “bundle of qualities,” the object itself seemed to disappear.”
The Making of a Philosopher
Simon and Schuster