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Hn...It seems like you've already given some of these options thought, but I'll spitball anyway.
*a solo run might be interesting, but if you're already refusing to do no-fusion, then maybe you'd probably wouldn't like that either. (Another user previously asked about the solo run, but was discouraged by others from doing it because it'd be too tough: https://www.gamefaqs.com/boards/961651-shin-megami-tensei-strange-journey/54375857)
* Maybe avoid hitting enemy's weaknesses. Or more simple: if you ever use elemental attacks, just use one type (ie. only fire, never force).
* Re: "demons (ie only light)" This might be fun. You seem to be suggesting only using very specific alignments of demons, which actually would be really cool. For example, If I were to do a run like this, I'd stick to Dark-Neutral, in which case I'd get access to types like Wilder, Vermin, and Fiend. If one really wanted to really restrict themselves, however, then they could stick to a single race, like the Fiends, who are only available at somewhat high levels and often (always?) require unlocking. Or one could use any sort of Dark demon if they wanted more options, gaining access to Tyrants, Raptors, etc.