I do not really know that much about Grav code base other than it is very extensible. But I was also hoping for a “standard way” to write things like that. But I can see you are right, and you know what you are doing.
Could maybe you take a quick look at: https://github.com/bleutzinn/grav-plugin-add-page-by-form/blob/507be4be7ad2b94a4b403c070b99f7c03566a8b8/add-page-by-form.php#L302
From this post: https://discourse.getgrav.org/t/get-form-data-select-input-and-add-it-to-page-frontmatter/4465
I guess it is an easy fix for some one who understands the logic.
Thanks