iTunes will not transfer / sync correctly (dotted grey circles)

I am experiencing some kind of syncing problem, the result of which is that, when I look at the Music on my iPhone within iTunes, many songs are "greyed out", with a dotted circle appearing next to them.

Some background: I created a rather large (400+) playlist of songs within iTunes. Many of the songs on the playlist were already transferred over to my iPhone, but most were not. I dragged the completed playlist (under "Music Playlists") over to the iPhone (under "Devices"), and while the playlist itself appeared on the phone, only the songs that were already loaded onto the phone appeared normally within the playlist; any song which had to be transferred from my Music library over to the iPhone appeared greyed out with a dotted circle next to the song name.

Subsequent transfers / syncs of music which are not part of the playlist in question have also had issues. For example, I transferred an eight (8) song album to the iPhone: on its initial attempt, iTunes only managed to successfully transfer four of the songs -- the other four appeared greyed-out/dotted circle when viewed under "On My Device">Music; however, when I manually synced the iPhone moments later, the remaining four songs were copied successfully to the phone.

One thing I noticed during this 2nd copying/syncing attempt was that, while viewing the iPhone 7 summary within iTunes, the capacity bar graph at the bottom of the iTunes screen fluctuated from roughly 30 GB Free pre-sync to 34.8 GB Free post-sync (i.e., the 2nd sync, which completed the transfer of the 8-song album). As the 400+ song playlist I have tried to transfer contains just under 3GB worth of music, I suspect the fluctuation in this amount of "free" space on the iPhone is somehow tied to the sync error, but beyond that I don't know what it means.

[Since I initially wrote the preceding paragraph, I have noticed that the free space on the iPhone has inexplicably changed to "30.43 GB Free", so approx. 4 GB of space has "disappeared" even though I haven't synced the phone; I'm sure this has something to do with my aforementioned syncing problems, though I don't know what it means.]

In any case, while I was able -- in two attempts -- to get the album to load correctly, these syncs had no effect on transferring / copying the missing songs from the 400+ song playlist onto my iPhone.

I am technically proficient and have scoured the InterWebs for guidance on how to fix this issue. Some methods I have attempted include:

• manually syncing the iPhone (which I have done repeatedly; while this method has worked for me in the past, at present it is doing nothing to remedy the situation);
• syncing over Wi-Fi rather than USB cable;
• deleting the playlist and reloading it;
• manually transferring the songs which comprise the playlist (individually or in batches) rather than using the playlist itself to trigger the transfer;
• restarted both the MacBook Air and the iPhone, with no apparent effect on correcting the syncing issue

I'm at my wit's end: having my Music on my phone is (almost) more important to me than the phone's telecommunications or Internet capabilities, but lately iTunes has caused me more grief (and wasted hours of my time trying to correct problems, such as having the wrong album artwork transferring onto my phone when I'd add songs) than it's worth. If there is a third-party app that works better / more reliably than iTunes, I'm prepared to jump ship, but for now I'd like to see whether someone out there in the Apple community (or Apple itself) can provide a solution to my problem.

In short, HELP!!!

Tech specs, FYI:

• I have a MacBook Air (13-inch, early 2014) with a 1.4 GHz Intel Core i5 processor and 4GB of memory (1600 MHz DDR3) running under Mac OS High Sierra, ver. 10.13.1 (the current version of Mac OS). I have the current version of iTunes (12.7.1.14).
• I have an iPhone 7 (model MN9H2LL/A), which has 128GB of storage capacity (approx. 32GB of which is available -- or roughly 25% -- though this "free space" number fluctuates, as described above) and is running the current version of iOS, 11.1.1 (15B150).
• I am using an original Apple Lightning cable to connect the iPhone directly to my MacBook Air for syncing.
• With very few exceptions (probably less than a dozen, though I'd have to go back and count to be sure), the songs I am trying to transfer from my iTunes Library to my iPhone Music app are songs that I personally ripped from CD: the number of songs I have purchased the Apple / iTunes store is dwarfed by the amount I have ripped from my own CDs, etc., so I doubt that DRM issues are at play here.
• For better sound quality, I'm in the habit of ripping CDs in iTunes Plus format (256kpbs, VBR) when adding music to iTunes, but I do have iTunes set to downsample /convert higher bitrate files to 128kbps AAC in order to maximize the number of song files I can have on the phone.
• My Music library is comprised of a few different formats (mostly mp3, m4a, or m4p) at various bit rates.
• I do not subscribe to Apple Music, and I have any Apple Music-related options de-selected on my iPhone or in iTunes.