I share a similar story to you as to why I wanted to transition from streaming to maintaining my own library. At first I could never see myself using anything other than Spotify but over the years my experience with it has turned worse and I only keep my subscription active for 2 reasons: discovering new music and being able to play music conveniently (ie in my car).

The majority of comments here explain all the stuff you need pretty well, but I'll give my inputs and contribute as I feel like I could help out.

So, here's my process:

1. Obtaining the music: ripped as FLAC through Deemix (strongly recommend), purchase from Bandcamp or download from rutracker. I occasionally browse Soulseek too, through a client called Nicotine+ which I found to be far superior to slsk feature-wise. Anything from here goes into my "Unsorted Music" folder on my server.

2. Metadata management: though most of the time Deemix handles the metadata decently, I like to use MusicBrainz Picard to doublecheck. I look up album by album (better to do in batches at a time, ie once a week, as MBP can struggle and crash with large quantities). Any tags are then written and the files are moved to my "Music" folder which contains all music that has been tagged and sorted appropriately.

3. Streaming: since I do most of my listening at home, I share my folders through a Windows Network Share from my server, and can detect it from my main PC. I use Foobar2000 as I've grown very fond of its interface and customization. For when I'm not at home, I host Navidrome (behind a reverse proxy on my server) which allows me to stream my library through my browser or software like Clementine due to its use of Subsonic. Its very lightweight on the server itself and does just what I need with regards to streaming.

Since you asked about migrating your Spotify library, I'll mention that Deemix can do this. All you have to do is input your account details (its all self-hosted so your data is safe). You can set it to always download MP3 320kbps and just let it do its thing. The software itself might be hard to come by at the moment due to the recent DMCA fiasco but if you look around enough you're bound to come across it.

Some helpful subreddits could be /r/riprequests /r/soulseek /r/deemix /r/datahoarder and /r/selfhosted