Definitely somewhat fun, I recognize some of these effects so I assume you used plugins for a lot of stuff.
Unsorted list of issues:
No way to return to default settings, this means you can't change the window screen color back to default because of how the plugin is set up.
The menu sound choice of not having a sound when opening it, but having a sound when closing is a bit weird. Might want to consider adjusting that.
The map is a bit annoying to get to, as are some of the other menu options. Might have been worth hotkeying the map.
The skill assignments are really weird since they give you controller prompts even on a keyboard. Keyboard controls are a bit awkward. Hard to attack, dodge, and use your skills. QWE would have been better as an option or what might have been better would be the ability to customize your controls.
For the UI, might want to consider adding a fade option.
When running, the default attacks show up behind you. One the of the weapons, the Astro Dagger, has no attack animation so you can't attack with it.
The boots of Magus have Lunge attached, but it isn't mentioned.
Some passability/layering issues such as in the tables/flowers in the library, the fish net and nearby object in the river area, the monument in the cave, and the chandelier shadows.
How is the old man able to just teleport between dimensions?
During one of the cutscenes, only some enemies show their nameplates, so it looks weird.
Mushroom sprites are messed up.
Items can disappear during cutscenes, maybe have an auto-pickup? Some later enemies seem to not drop gold and some of the earlier enemies seem to stop spawning.
You can't use a gauntlet with the spear, but you can use a gauntlet with every other non-Sol weapon?
The Major just goes poof when you fight Minor and only appears after the cutscene. Also, the bears don't aggro you immediately even though it would make sense to.
The tentacle monster looks a bit weird on the wood. Might have been better to have the fight on shallow water instead.
The volcano area is really dark for being a volcano full of glowing areas.
One of the maps looks really similar to another map, making it hard to recognize as a different map.
Enemies can spawn while in the menu.
The control guide disappears fairly quickly.
The spikeball is pretty OP to the point that Giga Shock isn't that useful because you have to sacrifice the HP regen for Giga Shock and the spikeball basically does about the same thing as Giga Shock except smaller range.
You can walk on the sky.
Enemies seem to only attack on four planes instead of 8, so you can easily dodge by staying diagonal.
Seems really weird to just randomly have a Japanese word.
In the post-game area, you are invisible.