It's a bit of a shame that the ending was executed the way it was because within it I can see exactly what they were going for but also why the show could have really used two maybe three more episodes to focus more on Mari and also how Lara doesn't really have a life outside of needing to find love and the ways it constrained her.
The bakery episode did get at it a little giving Lara a real interest outside of needing to find true love and the show could have really made use of more like it to highlight how she doesn't really have a life outside of her duty to marry and give her family the light. Though I do like how clearly the finale shows that her family just kind of doesn't care for her as a person and that true love as seen by the mermaids is just comphet and that the overarching narrative is about resisting familial pressure to get married for a family that only aims to parasitize itself on your love.
So while the ending is lackluster I do think it's moreso because the show could have used at least one more episode focusing on Mari, and another focusing on how constrained Lara is knowing that as the princess of light her life exists only for love.
But hey, maybe they saw the success of the anime and are just planning to make another season or a movie or something that actually cashes in on the one loose end of this story, Mari.