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OK WHAT
CAN WE GET THE FULL OST REACTION?

Added context: Sans typically has a lazy and sarcastic demeanor, so him coming out with the hardest boss fight was completely out of left field. Having a Mii costume is perfectly in character because he gets all his representation with none of the effort.

seeing someone learn the notes of megalovania fills you with determination

The cool thing about Megalovania is that it was not created for Undertale, and it did not debut in Undertale either! It featured in other projects Toby worked on before this, including a halloween-themed rom hack for Earthbound, and a webcomic called Homestuck. Each version is a different rendition, but the Undertale version is the most famous one.

"What is happening?!"
Someone with nothing to lose is trying their absolute hardest to end a threat to the entire planet.

10:40 suddenly a small white dog in a toy car crashes into you

Check inside that guitar for codes to the shelter.

Toby Fox is such a good composer, Undertales soundtrack is so so iconic, Deltarunes is very good too, I love all of the Toby Fox music

Fun fact. It was a fairly well known theory back in the day that Megalovania was NOT the theme for sans but instead YOUR theme. Supported by a couple of things. Firstly there is a song called "the song that might play if you fight Sans" which is hinted to actually be his theme. Particularly since it holds the motif for one of his brothers songs Bonetrousel. As you mentioned it also doesn't really connect to all of the lager motif web. Also I'm sure youve been told this is not the first rendition of this song, the first time it was in an earthbound Halloween romhack as the theme of the big final evil boss, latter in Homestuck it was used as a kind of theme to a character that can be accredited as the individual responsible for most of the bad things that happen not exactly a good person she really encompasses how bad a character can be and still be liked in a way, and so it makes sense to keep this song as one for a main evil or bad force. YOU are the villain in this moment. You've offed every monster in the underground, you terrorized and left carnage even when you were treated kindly. This is YOUR boss music not his.

Worth pointing out that a lot of the music in Undertale is microtonal! Bergentrucking and ASGORE are fantastic examples of this.

5:00 is when the actual reaction starts.

Your analysis on the Blues vibe of Megalovania is spot on. It's what ties it to Sans as a character, all three of his songs are in that style.
The song you first hear when you meet Sans is very goofy & simple. Kind of like how Sans appears on the surface.
The second of his songs is somber and contemplative. Giving you a look into Sans' true self.
And then there's Megalovania, which is the pure essence of vengeance.

I haven't seen anyone mention this, but the 'evil' route where Megalovania plays is actually weirdly hard to actually do if you don't know what you're doing. Yeah, the good route is basically if you refuse to kill anyone, the neutral route happens if you kill just ONE person. But the evil route is where you not only have to kill everyone you meet, but run around in each area and grind battles until enemies just stop showing up because there's literally no one left alive in that area. The music for that area, when that happens, disappears. It's exceptionally slowed and distorted versions of other songs, there are alternate versions of boss music. It's actually rather eerie, because you're intended to play through at least a neutral route first, so you know perfectly well what things are supposed to sound like. And they just don't; like the game itself is punishing you for choosing this route by stripping away some of what makes it fun.

Cant wait for him to listen to Amalgam

"U GONNA HAVE A BAD TIME, KID"

Cool thing about MEGALOVANIA - you might not hear it on your first attempt of fighting Sans (boss). Undertale is a turn based RPG, and in every fight before your turn is first, but Sans attacks you first with quite hard attack, which for inexperienced player almost always leads to death. Only after you complete this attack MEGALOVANIA starts playing, and the thing is - all bosses before on this evil route are one-shoted by player, besides Undyine. So again, player can try that only to see, that Sans dodge attacks, something that only he can do. It's VERY surprising fight on the first playthrough, and both music and how it playes made Sans and MEGALOVANIA most popular Undertale character and piece of music (with the only contender being Hopes and Dreams).

Everytime I feel like I've overheard this song at this point, I listen to it and get hype all over again

If your looking for another really good undertale song I recommend the track titled UNDERTALE, it's a favorite of mine.

people often like to joke that Toby wrote 5 songs for Undertale and then reused them over and over in different ways for the whole soundtrack, but while that is partially true they never seem to acknowledge how much skill it takes to put so many recognizable motifs and themes together and have it STILL SOUND GOOD in different keys and rhythms across a pretty wide spread of areas and characters.