TRANSFER music from iTunes to iPhone (ie one way, not sync)

I'm sick of iTunes sync messing up my PC iTunes library. Today, by syncing my iPhone, 80% of my playlists were deleted from my PC iTunes - because they weren't on my iPhone. So now I'm having to restore the whole library again from backup.

I simply want to have a master library on my PC iTunes and then transfer selected playlists to my iPhone, iPad and iPod.

Why, oh why, I spend so much money on Apple I don't know - it really is a very shoddy offering all round. I gave up on iTunes Match because of the chaos it brought to my iTunes library and have already given up on Apple Music for similar reasons. I simply don't trust Apple anywhere near the management of my precious music!

So all I want to know is: how can I TRANSFER (not sync!) music from iTunes to iPhone ONE WAY. I just want to say "here you are iPhone, here is the music from my iTunes master library that I want you to have...", "here you are iPod, here is a different set of music from my iTunes master library that I want you to have...", etc.

Can anyone help please?

Thanks!