Syncing music from iTunes on my computer to my iPhone (MP3 files)

[NpXX]

Plan: Premium

Country: USA

Device: iPhone XS Max and MacBook Pro (late 2013)

Operating System: iOS 13.1

Using the newest versions of Spotify on my Macbook Pro and iPhone

This has been bothering me for years now, so I've refrained from even trying to even get to the bottom of it until now. Spotify's "way" of integrating music is so annoying and frustrating, that it's making me want to switch to AM simply because of how unintuitive it is (I would use a lot more colorful words because I've wasted so many hours over the years trying to figure this out and still get unsatisfactory results.)

I am connected to the same wifi on both my Mac and iPhone and have a library of "unreleased" songs from artists I like and want to import into a separate playlist on Spotify to be able to listen to on my iPhone as well. 

Every time I go to local files on my laptop to move songs onto this playlist (again, on the laptop) it seems to work okay, which makes sense - the songs are downloaded onto my computer so of course, they'll play there. I have the playlist set to download these songs, and on my laptop they do. However, when I open Spotify on my phone the songs either aren't there, are there but won't play, and the album art is always missing from nearly every track (this is also the case on my laptop for most tracks even though I have album art for them). I only use .mp3 files so I don't know why they won't at least play (they play on iTunes on my laptop and I've made sure they're .mp3 files). I've tried turning downloading on and off so many times but nothing works. I've tried making new playlists and starting totally fresh and that hasn't worked. I'm getting so frustrated at how stupid this process seems that at times I just want to throw my computer at the wall. And what do the dimensions of album artwork have to be so that they even show up!?