mtg removal cards Lorwyn Eclipsed: Guide and all Auras you own attached to attacking creatures your opponents control Remove Soul · Classic Sixth

There are a few hyper-specific cards that care about permanents transforming, like Corruption of Towashi and Cult of the Waxing Moon, but none that are too competitive in Commander

Effects like the Historic banned Spreading Seas can fix this and, since Islandwalk is a recurring mechanic across Merfolks, it is probably worthwhile running some effects like that, making Master of the Pearl Trident an absolute menace

Of course, good will alone wont win you the game, so Font of Mythos will only be a good fit in decks that know what to do with the extra cards they draw (or how to punish foes for being flooded with cards)

But yeah, I like Basandra in a Boros , Mardu or a dedicated goad deck

Now, onto the lands themselves

The best place for this card is alongside one of Magic's mono-red treasure-making commanders, where the restrictive color identity means you don't have so many options of cards to choose between