Mark's solution as outlined, is pretty foolproof and reliable.
I continue to use a NAS (Synology), but its native apps have expired over time and network settings tend to stop connecting and have to be reset, when you "just want to play the damn song, already". - so I like the idea of an external hard drive in combination with a dedicated laptop.
on the iPhone itself, you may wish to try "Foobar 2000" (which allows you to navigate via file folder and may bypass the syncing/file-tags problem). "VLC" can be used as well. Neither are great interfaces for playing music, but I find they at least let you have more control over file location either in iOS or Android platforms. For example, you could set up a local folder in iPhone "Files" app, to download and store those exceptional, but favourite live show tracks, and then add it to Foobar as a media location, or navigate to it, with "Files" itself, to play the music in VLC, or whater. Then use the default iPhone "Music" app, for mainstream listening.
I have not found a single solution that covers all bases and sources. I'm sure there is criticism over the various apps and approaches, but for me I have had to settle with a hodge-podge of choices, depending on the source or type of music. I'm still symied how bad "newer" (players/streaming) apps are at navigating music collections/folders.
I have colleagues that continue to happily use older mp3 players to have full control over their music management (refusing to stream or buy into any particular tech ecosystem). There's no harm in going that route, as you originally hinted, Sumant.
As total aside, I had minimal success with PLEX b/c the player would not work with certain filetypes. I went with Emby (after trying SEVERAL other solutions): it's only downfall is to not always pick up on newly added content, but as long as I re-index the NAS manually, on its separate indexing service, the files are found properly and play well.
I hope this potentially saves you some time; I totally share your frustration.
Cameron