"So smashers and spirits + universe = one shot by Galeem, but somehow smashers + some spirits are supposed to be on par with him. That doesn’t make any sense"
Smashers that have gained power from the skill tree + Spirits that have leveled up significant since being turned into spirits yes.
"Also there is literally someone on par with him, Dharkon, and both him and Dharkon are trying to kill each other 24/7, while the smashers are stealing their spirits thus causing their power to flux all over the place. I find it way more likely Galeem and Dharkon just accidentally kill each other, rather than the smashers being able to win despite how completely illogical it is." First I am pretty sure that it is because they were losing members of their army they lost territory not because they lost power.
"We should treat them scaling to Tabuu in the same way we treat people in JRPGs progressively eventually scaling to the Final Boss. In other words, I agree with Tabuu, because we have literal evidence they can at least somewhat hang with Tabuu, Master Hand, and comparable Bosses." There is evidence they grow in power during the world of light.
"how do we scale the fighters pre-Great Maze?" Primid? Maybe
"we already know the characters are somewhat comparable to Master Hand, as he sees them as worthy foes (mentioned in some of his trophies) and the fact they are confident in taking at the very least of his clones head-on at the beginning of World of Light (throw him being a recurring final boss for support)."
I mean he goes head to head with the fighters amped by spirits and his trophy in brawl says:
A being tied to the link between this world, where trophies fight, and the world beyond. The meaning of his existence is unknown, as are his goals, but he seems to have obtained (and kept hidden) a power that borders on absolute. He also seems to feel a certain joy in challenging chosen warriors who've claimed many victories. He waits even now in Final Destination.
"Plus, again, we have no clue if the smashers actually really do any damage or if Galeem and Dharkon are doing all the work. They are specifically on par with each other and are trying to kill each other."