Okay, this is my very first review and while I am already the artist for another game in this wonderful Jam, I'll try not to hold any biases and will try to stay positive with this as this game has a lot of positives to it.
Advent of the Cosmos is a bit deceitful at first. Initially I thought it was just going to be an RPGMaker Horror game only using the custom art as a small confetti over RTP sprites using default tilesets. But then there were things that surprised me when playing this blind with my SO watching to make sure I haven't gone mad.
1] The Point and Click Adventure Bit - I love this bit initially, however I am more used to using the WSAD controls in my RPGMaker games so it took some time to get used to clicking my mouse across the screen. Either way, I love how this game made you explore and investigate initially in the human world. I feel inspired to do something like this in a future game I do however will hope to still implement keyboard controls for those that are usually comfortable with it.
2] The most RPG-ish part of the game is [SPOILERS] however, I will add the [SPOILERS] were nice. Why I am trying not to spoil anything here is while the game DID crash on me in this part of the game during the climax of the game [that's why Gameplay got points docked for me] is that I love this supernatural world they went in. It's like a twisted version of one of the special maps in the 2D village in Dragon Quest XI but you control characters that give me mad vibes to a Life Wonders game [Tokyo Afterschool Summoners and LiveAHero for those that are not familiar with mobile games with turn-based combat, which seem to be Life Wonders' bread and butter]. I won't say anymore though, my only criticism is that Gui's human form made me read him as a bearded Son Goku archetype more then what his actual supernatural form looked like. It isn't a bad thing of course. It just made me keep on guessing.
3] The Dialogue - The best part of the game. Marcus and Gui both read as fun people to watch [and possibly ship, even if those that don't ship it can read Gui's cute little 'taking Marcus out to breakfast after this is all over' as something fun to read aloud. Though, I wish I could say the same for the NPCs as while the humans are passable, the [SPOILERS] range from cheeky to something ABSOLUTELY cursed and in the 'just scrolling through Tumblr and found a cringey furry' kind of way [but don't worry. What happens after that interaction is actually funny].
4] The Art - Like I said, the Art surprised me. The Custom Art showed up more than I was expecting and that is a pleasantly good thing. I've often had people give RTP Chibi sprites that we got with the default assets in RPGMaker MV and MZ a bad rap, but here, they use the default assets as a sign of how mundane the human world is compared to [spoilers] so I find it here using the default sprites here for the first time as a positive due to what came afterwards with [spoilers].
5] The Puzzle - There's a maze puzzle in [SPOILERS] which while was simple to navigate for smart people, made my gameplay experience take longer due to my incredibly bad memory. I hope people have more fun with it as well as you get an important item for the battle after it!
OVERALL: I would give this game a 4.3 out of 5. Please keep it up! And I hope to finish the game properly very soon!