why isnt' iTunes syncing all the songs I checked?

I have Mojave - I can't upgrade past that because this is as far as compatibility with MS Office 2011 for Mac will work - or so I've read. I was doing fine with Sierra until I made the serious mistake of updating iOS to 14.8 (from 14.7). All of a sudden none of the songs on my iphone would show on my computer so I could manage them - an Apple tech told me that 14.8 wouldn't work with Sierra - if I had known that I wouldn't have updated anything! But it was too late - Apple wouldn't let me go back to iOS 14.7.

So I updated the OS until I reached the limit for MS Office for Mac 2011, which is Mojave. I can't afford to give MS hundreds of dollars for 2016 nor do I want to since I have lots of very valuable docs I don't want to jeopardize - and I trust Apple and Microsoft about as far as I can throw them.

OK - so after updating to iOS 14.8, and none of the hundreds of songs on my phone showed up on my computer when the phone is hooked up, I thought updating to Mojave might fix that, but it didn't. My iTunes version is now 12.9.5.5.

So today I spent all day going thru the thousands of songs on my computer, checking the songs I want to sync from my computer to the phone, to replace what's on the phone. Supposedly. I made sure to tell iTunes to manage music manually and sync only checked songs.

Well, after all that, iTunes synced 66 out of the hundreds of songs I checked.

What is going on? I understand that with the latest OS iTunes doesn't even exist anymore. I'd be fine with that except for the Office problem and because iTunes has been a nightmare for me ever since I started using it more than 10 years ago. Please don't tell me to update to the latest versions of iTunes and Mac OS and iOS. I just explained why I can't - and I will never update to iOS 15 because of the spyware - I will dump this phone and go back to using a flip phone for calls and texts, and use the phone only for the music I have on it now. Can you tell I'm infuriated?