For example, if one might describe themselves as an "occasional writer;" or, identifies as a writer, yet only "writes occasionally." Would "sometimes-writer" be a more laconic way to express this?
This is a personal take on the question (nothing even remotely authoritative), but I think the three usages mean slightly different things.
I take a description of an individual as an "occasional writer" to mean that they undertake many actions or activities, one of which is writing. A manager of a department might have to write descriptions of accomplishments or proposals for future projects. They do not write "full time" but they do engage brief episodes of writing as a regular part of their duties.
I take a description of an individual as one who "writes occasionally" to mean that they are known to write but only infrequently. A board member might be called upon to write a grant proposal when the executive director is otherwise occupied. They do not write "full time" and when they do write, it is comparatively rare and typically of moderate duration.
I take a description of an individual as a "sometimes writer" to mean that they have written (more-or-less fulltime) in the past and that they might do that again in the future. A football player wrote a memoir a decade ago, published a sports-oriented novel two years ago, and talks about a follow-up memoir to be written in a year or two. They do not write all of the time but when they do write, it tends to consume most of their energy.
I value precision in writing and in word choice. If the context of the usage is a casual comment made by one character about another, then perhaps the word choice is less important. If the context is a narrative description of a major character, it is more important.
And, for the record, I have written occasionally, been an occasional writer, and have aspirations of being a sometimes writer.
I don't think 'sometimes writer' is correct. The final S is the problem. Compare 'once upon a time' with 'once upon a times'. 'Sometime writer' means at some time I have been/am a writer.
If there is no reason to ditch 'occasional' then I'd stick with that if that is an accurate description of bursts of creativity.