How to change music and playlist on iphone, not apple music

I currently have the same music and playlist on iTunes on my PC and on my iPhone. All my music is a combination of CDs, non purchases and iTunes purchases. I do not use the Apple Music subscription. Most iTunes purchases I have I make on my iPhone rather than iTunes on my PC.

In the past when I sync my phone with my PC I have iTunes set up as manually manage music I believe. I never really understood what setting I should have and lately I’ve been running into sync issues such as greyed out dotted circles next to song and songs mainly iTunes purchases not syncing properly.

I have recently been deleting songs, some of them iTunes Store purchases on my phone but not on my PC. I also want to change music in my playlists. My concern is if I try to sync my phone with iTunes on my PC everything will be messed up and not sync properly. This may also add to the issue of the greyed out songs with dotted circles and some songs that won’t sync.

Does anyone have advice for the best and easiest way to make changes to my music? Is it best to use iTunes on my PC, makes all the changes I want and then somehow delete all my music on my iPhone then sync? If so how do I delete all music on my iPhone? Also when I do sync my music what settings should iTunes on my PC be set to?

Thanks.