Warning: this post may contain spoilers
A bit late to the party. How did you feel about the show?
The retro 90s artstyle is definitely the highlight for me. Really gave me fluffy feeling of watching anime when I was (more) little.
Though that look was not a straight copy but got updated with some slight modern touches,
like how the story is also an expanded version of the original fairy tale with a more contemporary view about love.
Now romance is not my strong suit so what I could glean from the story is pretty rough. How I interpret the ending is something like the beginning of a long-distance relationship, Lara went out to explore more of the human world she loves, while being secured that the connection she formed with her loved ones back home would remain strong.
A running theme of the story is how there's no one singular "true love". This was stated most explicitly when Lara saved Mari from the fish dagger, she yelled about how she didn't need true love but only to be "in world with you [Mari]". One way to interpret that Mari was her true love, but I think it's more interesting if she wasn't "the" one, but that didn't matter because Lara cared about her enough to go through the struggle for her, and that was enough to dispell the curse.
Another point for this being how the mermaids' light power worked is in Grace flashback, we was shown that Rigmore loved Rowan enough that her light could keep the kingdom alive as what everyone wanted, but she also loved Grace enough to come rushing and sacrificed her life for her. So which love was more "true"? Of course both were, and that was okay. Lara desperately searched for an ultimate love but maybe what the light required was just that she loved something/someone strong enough.
Earlier the show hinted at it being about multiple forms of love when Mari's brother complained about how he would never be able to find love, but his grandma reminded him that she loved him too (to which he said that's not the same). The show made it clear enough that her affection for Luca was genuine (she mentioned feeling a sense of warmth when being with him that she didn't feel with other people). It wasn't necessarily that she loved Mari more, her feeling for Mari was just different. It's most straightforward to see the ending as yuri but I see them behaving more like sisters with each other. Beside, Lara is a grown-ass adult no matter how you interpret her age while Mari is still a highschooler so making a couple would be really weird.
Another reason why I think Mari was not "the one" of the story is the fact that we're introduced to her being curious about human artifacts first and foremost. If being forced to pick one thing, I'd say her love of human culture was the "truer" one and her infatuation with the prince (then Luca and Mari) are partly because they were a part of this culture she was enamored with. (This is how I felt about the original fairy tale as well. If the prince was meant to be her true love, it could have been more effective to introduce her starting out hating humanity, but this particular prince changed her and won her over.)
And so this ambiguous ending worked for me. She was finally free to live true to herself and explore what she loved most which is human culture, not just being with any one particular human.
Something I'm also curious about is the red/blue color scheme for the mermaids. There must be some significance to who was chosen as red or blue but I couldn't quite tease it out yet.
Someone said Sayonara Lara sounds better than Goodbye Lara so that's what I go with for the title :)