Syncing doesn't copy all songs and corrupts playlists on my iPhone 7

Not getting any replies in the "Using iPhone" area, so trying the "iTunes For Windows" topic.

I use PC iTunes 12.9.5.7 to sync a single playlist "On" to my 128 GB iPhone 7. It contains 5,893 songs in mostly ALAC format but some MP3 as well. I've been having iTunes convert to 128 Kbps AAC, but after noticing there's now a bitrate option, I wanted to go up to 256 Kbps. I removed all songs, and when I resynced, there were several hundred errors of the form "iTunes could not copy xxx because the file could not be converted." I've seen this before in 9 years of using Apple products, and I was hoping syncing again would set things right. However, not only do the several dozen to hundreds of songs needing to be synced change with every sync, the PC playlist is being duplicated to "On" and "On 1", and they and the original "On" are losing songs. Again, these are playlists on my PC belonging to PC iTunes! Fortunately, I have backups, so I didn't lose anything, but it is crazy that syncing can silently lose songs in playlists, creating problems I would only notice when playing an album and wondering about that missing song or two.

The good news is, I may have found a workaround. I'd been syncing over USB, and the errors reliably started to accumulate within a couple of seconds of the start of the copying. I've never once synced over wifi, but I decided that since I can't get my music on my iPhone the wired way, it couldn't hurt to try wireless. Three hours and 1427 songs later, there have been no errors or playlist duplication or corruption. It's a much slower process, but it does seem to be working.

This is just terrible. PC iTunes has always been less than reliable for syncing music. With my iPod Touch 4th and 5th generations, there were copying errors, failures to complete sync, etc, but as with the iPhone, it all worked when new. Indeed, the USB connection used to work fine for my iPhone. I don't think the hardware is at fault, because copying a several hundred file subset of "On" without converting to AAC went fine. I'll update the thread with the final outcome of this nonsense, but does this sound familiar to anyone? This shouldn't have been happening in 2010 with my iPod Touch, and it's beyond absurd that Apple is still screwing this up.

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OK, 20 hours later, and the wifi sync is complete. There was a notification on my iPhone telling me 8 songs didn't copy and to check iTunes. iTunes had no information whatsoever on this, so I went to sync again, and it proceeded to copy 64 songs. I checked my "On" playlist, and it was missing 8 songs. So, I deleted it, duplicated "On - Master" to a new "On", and I resynced. This time, it copied 4 songs with no indication anything was amiss, and iTunes indicated 5,893 songs were on my iPhone, which is expected. However, when I checked General->About->Songs on my iPhone, I found 5,889 songs. Sure enough, syncing tries to copy the same 4 songs from here on out, and when I look for them on my iPhone, I find they are grayed out, and I get this error when I try to play them, "This song is not currently available in your country or region," which I started a thread on 2 days ago, also with no replies so far:

https://discussions.apple.com/thread/250391093

Again, these are songs I ripped from CD myself, so the error message is completely bogus.

In summary, syncing over wifi instead of USB was notable for the following:

1. No playlist duplication on my PC. No "On 1" or duplicate "On" playlists were created. However, the first sync did delete songs from the "On" playlist on my PC. Fortunately, I now understand the iTunes requirement that you keep a master playlist that you can duplicate when iTunes screws it up for you, and I was able to recover without dragging out a backup.
2. My music did copy over with many fewer failed songs, which did copy over on subsequent syncs. With USB, there were hundreds of failed songs, and the set of failed songs kept changing with every sync and never stabilized as iTunes kept spontaneously creating duplicate playlists on my PC and modifying the original playlists.
3. Four songs out of 5,893 are grayed out on the iPhone and are unplayable for the bogus reason described earlier. This is a stable list of songs iTunes copies on every subsequent sync, never giving a hint that anything is amiss.