I basically design the fully, or almost fully in a document where from I split everything that has to be done in small independent tasks: Player controller, enemy ai, lights switch, etc.
My basic rule is: 'it can be a little janky/buggy as long it doesn't negatively effect the gameplay/experience.'
For example: If the enemies are too fast or shoot with incredible good aim, performance issues, bad UI, etc. That something i fix. If the enemies sometimes get stuck trying to walk over decorations or their AI is so lame they just run straight to their deaths. That's something i often don't fix because the Jankiness more or less make the game more easy to the player... and sometimes is fun to see stuff like that. I mean I may fix those stuff, but only if i see it necessary. IDK, i just test things like my life depends of it and if i am happy...ish with the overall result, i just keep it like that.
That because I am a lone-dev and can't spent all my time and resources overpolishing everything, specially if we consider that many things that takes a lot of effort are barely noticeable in the final product, or to put in other words. Any player will actually appreciate how much time, effort or investment took you to create every single aspect of the game, they will see something that probably took you months or years and barely pay attention to it. Or maybe, arely, some will say. 'oh, that's nice' and continue like nothing. Not because they are bad people o anything, but because the only person that understand how long and how much took you to polish every single aspect of the game is the people directly involved in it. In this case, you. .... where was i? oh yeah. Just, go easy on you. It's okay if some things are not perfect, in the end, if 'it's just works' that's good