I can't help but feel that this completely undermines your point - Vampire Survivors is bashed together using rudimentary knockoffs of sprites from games from the 1990s, in an engine which barely supports the idea of particles let alone proper visual effects.* It is the gameplay that carries Vampire Survivors, not the aesthetic.
Game feel is of course essential to producing a good game all-round, but a competent game designer can and will tell the difference between a good game design and a bad one, way before polish and juice are layered on top.
*I don't say this as a criticism - Vampire Survivors is fantastic - but the idea that it's propped up by its look is just daft.
- Make the game legible. A good example would be to reduce the whole game to boxes, you still need to differentiate things, so you might want to color them. Aesthetics in support of gameplay to make the game understandable.
- Add 'game feel'. This is where audio is especially important as you tend to notice the lack of good audio rather than its presence. But also 'juice', animations and what not all layer in.
- Support the fantasy. The name Vampire Survivors carries expectations that boxes do not match.
If you've ever done a lot of playtesting with your target audience one thing you'll find is that missing these elements gives you much worse feedback. Most notably legibility because it's so integral to being able to play a game but the others as well.
Game designers to an extent can get past this but it's still an attempt at extrapolation which is necessarily less concrete. Also if you're new to making games then you're going to make it harder to judge your own work.
The good thing about Vampire Survivors as an example is it shows that you don't need to do much but enough.
The dev team had just come off developing Titan, a cancelled MMO where they had trouble making the core game loop engaging after seven years of development, so they had a lot of motivation to start small and make something good first, then polish later.
You might spend a ton of time and money on art and polish only to suddenly realize your game isn't fun at all. Many such cases.
Having watched/read a few things about the white boxing stage the general advice is to put in as much polish as you need to do that and no more. If you're trying to prove out jump mechanics literally just some boxes for platforms and a sphere as the character is enough. If you're making a stealth game then you'll need some lighting in your level because it's a core game mechanic.
Most indie projects die before getting there.