I think if you add savelevels=true in serveroptions it'll save to the file.
Having an NPC for each tile would probably lag, though I haven't tried it. I'm not sure if you can do Terraria in Graal without lag but you can try I guess.
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Testbed user: I figured since I can never find any scripters it was time to take desperate measures...and...TEACH MYSELF 0.0