I think implementing submenu for Khmer based on Thai submenu would make mose sense with some few additions. Letters, Marks, Numerals, Symbols (for lunar dates), Punctuations, Others.
Since writing Thai and Khmer is based on the same rules of consonant and vowel placement in Thai (almost identical with one key difference) basing the submenu on what is present for Thai makes total sense to type designers.
The Khmer script is divided into these sub categories Letters, Marks, Numerals, Others, . In Glyphs app, Letters in Khmer are single character consonants and independent vowels. Independent vowels are just seperate graphical representations of vowels that are rare, but commonly used in old words, Pali, Sanskrit, or special significant technical terms. All vowels in Khmer, called Marks in Glyphs app, need to attach to a consonant. These vowels are called dependent vowels for this reason. They cannot be written or read without a base consonant.
Seperating the Mark from the Letter make sense in every context. Although Khmer do use combining marks that behave just like latin diacritics, they function nearly the same way as vowels except the fact that they always appear above the base consonant (Which I think is universal in all Latin based scripts. Vowels in Khmer can be placed above, below, to the right or left, and combination of each direction around one base consonant.
Numerals - Khmer has its own numeral digits 0-9, plus special divination lore numerals 0-9, each with its own specific glyph representation.
Others - I'd keep these for punctuation marks.