Thank you for your clarification Mike and also @Adventine and Bob for your inputs!
Unfortunately, the online form does not allow for a self indicated job role, it only allows the job descriptions USCIS has set for that tab, although I agree this would be the best approach if there were such an option (I expected there to be an option to just type it in freehand too, but alas, no).
For anyone curious, I briefly tried the descriptor "Occupation not reported/unknown/invalid" in the hope it would prompt a comment box for further explanation, but it doesn't either, unfortunately (I would not choose that option by itself, as the general field for "education" is available and I expect choosing this answer by itself would for sure raise red flags which is the exact opposite of what I want).
It does sound like the consensus is that there will be room to explain/clarify at the interview though. Keeping that in mind, I think I'll go ahead with first and last day worked and assume it won't be a detrimental issue, it seems at least it shouldn't be, as long as I explain it further at the interview.
(That said, I am aware I could redo the whole application by hand and file via mail as that would at least allow for a self indicated job role with the word "substitute" in it, but I strongly prefer to file online after reading through some threads and decided it is not worth agonizing about it - just for anyone in the same position, know there is a way to circumvent this issue altogether, if you want, by self indicating your job role on the paper form ).