I would imagine a solo run on an RPG would be difficult though the obvious obstancles would the Garuda who could hard to kill with just Syle (trying to fight it with Alyssa and Ein was impossible for me) and the Paslime who has an attack that deals around 1/3 of Syle's Health. Lastly Syle may have to use Shards to get past obstancles until he/she gets those spells since you won't have Alyssa, Ein, or Jin to take care of them for you. Also I assume the party earns less EXP with 2-4 members (I think previous versions had the same EXP distribution regardless of party members) but you mentioned that Grades (Levels) don't give much stats since most stats are from equipment similar to Terraria.
I would wonder how some scenes would look with just Syle (i.e., without other party members or NPCs) since it would look wierd without Syle speaking/thinking.
When I play 0.8, I'll try to comment on it. As for macOS, I'm not sure how Lawmage Academy will work on Big Sur due to Apple deprecating OpenGL support as mentioned here on RPG Maker Web (Windows and Linux are unaffected and should still work).