I think that you are asking some valid, but extremely complicated questions to even define/ask properly.

It starts with the definition of a "language". What if we consider cases like Chinese or Arabic where many different spoken languages arguably map to the same or somewhat similar written language?

What if the language learner already knows, natively or otherwise, a language that's distinct from the language being learned, but helpfully similar on either the written or the spoken side?

Suppose I'm reading the English sentence "It's December 18." Shall we count "1" and "8" as two letters, or as one composite sign whose meaning is "eighteen(th)" or even "the eighteenth"? And what about "D" versus "d" and what about the period character after the number (possibly mapped to an indicative mood pitch contour)?

What if a writing system has an extremely regular (shallow) orthography, but each letter corresponds to a syllable, rather than a "sound"?

That makes sense! And I’m happy to constrain the question if it makes it more appropriate / answerable.

I think the root of what I’m curious about here could still be addressed by considering 2 languages with a single shared alphabet, both of which have phonetic writing systems, and considering how individual phonetic symbols (IPA or similar) map to orthography (excluding digits, numbers, and mathematical symbols - the phonetic alphabet only). Perhaps only vowels as well, since this may still be a huge question.

I struggled a bit coming up with an appropriate summary for this one since I’m trying to ask about « is this studied / what is known so far » more so than a definitive answer on the relationship between orthography, phonetics, and learning for every language / learner.

"Orthographic depth" is the concept that lets you compare a language with another language. Different researchers define it differently and they rarely offer any numeric values for individual languages.

When learning a second language, however, any prior linguistic background trumps everything else.

Make your choice:

Either look for studies of small children as they learn to read and write, many of them in their native language; this way you might minimize or standardize their prior language backgrounds and learn something about their respective languages.

Or you could focus on adult learners, but then the ease/difficulty of learning is not determined by features of the target language alone, but rather by how the target language compares to languages previously known to the learner.

For the latter, you could perhaps start here.

I see - thank you for clarifying, the link is helpful. Would it make sense for me to close / delete this question?

Asking since it seems like I made some poor assumptions in forming the question (and it may not really be answerable as a result).

I can probably follow up with a more specific (and more answerable) one after a bit more thought / research. But in this case I think my curiosity exceeded my ability to define the question well.

Even knowing that Orthographic Depth is the proper term is a huge help in finding relevant research, so thank you!

I'd appreciate if you leave this question open because I would love reading any answers that may arrive, even if that takes time.

Asking additional questions as separate question posts makes perfect sense, too.

Ok! I’ll leave the question up - I appreciate your input Jirka!