Confusion about syncing Apple Music library

I've spent over an hour here trying to answer my own question and I'm still kinda in the dark, so apologies if this has been answered before.

I have over 41,000 songs in my music library on my MBP. Overwhelming majority of those are ripped from CD's, though a good number were purchased thru iTunes/Apple Music over the years. I've got several playlists created from all of this music. I subscribe to both Apple Music and iTunes Match (and am aware of the song limit under Match).

I recently got an iPhone 15ProMax and have (finally) more than enough room on the phone to have all of these songs live on that phone so that I don't have to be online to listen to anything. I plugged my phone into my MBP via USB and synced my entire library successfully to my phone. During this sync, I had "Sync Library" on both my phone and MBP turned off. My iPhone XR that I just replaced did not have room to store all of my music locally, so I had sync turned on in both places and it worked well (as long as I was online). I was able to add songs to a playlist on either device and it kept everything in sync on both devices. Same with buying music through the app, it would appear on all my devices.

So here's my question(s):

If I leave Sync Library off on both devices, would adding a song to a playlist on either device update to the other devices? Same question applies to purchasing music on either device.

If I turn Sync on, will the music that now resides on my phone be removed from there and replaced by a streaming version of the library that is on my MBP (that requires an internet connection to listen to)?

Is there a particular advantage of leaving Sync on or off in my situation?

Is it possible to have music stored locally on both devices, yet maintain synced playlists and purchased music in both places?

And lastly, do I need to subscribe to both Apple Music AND iTunes Match, or can I drop the Match?

Thanks in advance for any advice. Most of what I found in these forums applied to some of my questions, while other info here contradicted itself.

Kind of hoping to have the best of both worlds, but unclear if that's even possible.