A lot of the reasons for these spellings is actually history. For example, the distinction between broad and slender was originally a purely allophonic distinction, determined by adjacent vowels, so the use of silent vowels to distinguish broad and slender was derived initially from vowels that were originally present, and subsequently extended where needed
The use of bh for /v/ wasn’t someone simply copying the Latin ph. In fact, it originally wasn’t even spelled that way at all! The traditional Gaelic script used a dot above the letter rather than a digraph with h
This reflected a sound change that was also initially allophonic (and indeed, there was a period when it had become phonemic, but wasn’t yet indicated in the script, so b could be used for both sounds, and then that diacritic was adopted to distinguish the new sound, and then, of course, when new words were borrowed with the /v/ sound they logically used that same convention; using a diacritic was convenient since it made the change in spelling very minor and you could even add the diacritic to texts that didn’t make the distinction). When the traditional Gaelic script was replaced with the modern script, the dot diacritic was also replaced with a following h. That part was probably influenced by the use of consonant + h diacritics in other languages (especially English), but the fact that /v/ was indicated by an altered b was already there. That diacritic is still used, though, in contexts where the traditional Gaelic script is used
Because that historical lenition process is reflected in the grammatical phenomenon of mutation, the use of -h or the dot diacritic is quite useful, since an orthographic alternation between, e.g., an initial b- and an initial bh- preserves the relationship between the words much better than if it were something like b- and v-. Especially useful since several of the lenitions become homophones, if you’re looking up an unfamiliar word that occurs in lenited form, using a consistent orthographic representation (dot above or h) lets you easily figure out what the base form is. Dh and gh are pronounced the same, for example, but the use of that convention lets you know that if you encounter a word starting with dh- in its lenited form, you’d look it up in a dictionary with d-, and likewise gh- you look up under g-. If they were written the same way, you’d have to guess which letter the base form had
EDIT: Actually, I made an error on bh, although I was still correct about it not being a simple copying of ph. Quoting Wikipedia “In Old Irish, the overdot was only used for ⟨ḟ, ṡ⟩, while the following ⟨h⟩ was used for ⟨ch, ph, th⟩ and the lenition of other letters ⟨b, d, g, m⟩ was not indicated, lenition being generally understood due to word position (somewhat like in Danish). Later the two methods were used in parallel to represent lenition of any consonant (except ⟨l, n, r⟩) until the standard practice became to use the overdot in Gaelic type and the following ⟨h⟩ in Roman type. Thus the dotted letters (litreacha buailte “struck letters”) ⟨ḃ, ċ, ḋ, ḟ, ġ, ṁ, ṗ, ṡ, ṫ⟩ are equivalent to letters followed by a ⟨h⟩, i.e. ⟨bh, ch, dh, fh, gh, mh, ph, sh, th⟩.“
Either way, though, it’s still a representation of a sound change in Old Irish
stormborns
ellievsbear
pixelskylines
turnnoffyourmind
dailybridgerton
areudirty
