That is, how would I go about converting the word “spelling” (as in the spelling of a word) to an adverb that actually sounds right in the blank of “_____-inept”?
I’m pretty sure “spellingly” isn’t a word, and likewise “spellatically” doesn’t sound remotely close to any actual word (I suspect in both cases that it’s something to do with the fact “spelling” is a gerund, not an actual noun), and I’m not what actually does work.
As always, all help, constructive criticism, and other constructive input is welcome.
You’re right that “spellatically” does not work; it’s a twisted mess of a non-word. I don’t know of any way to derive a fitting word from the word “spelling” for this particular context; I think you’d have to either rephrase to simply “bad at spelling” (“bad at” is the same number of syllables as “inept”, so this is really the most concise way I can think of saying this) or use an actual Latinate/Greek-derived term like “orthographically inept”, or if you’re feeling really fancy or pretentious, the noun “cacographer” (bad speller).