I FIXED IT. I know a lot of people probably don't want to hear this, but the time nudge issue (the three rectangles popping up in the top left corner) is definitely indicative of a client-side issue. But it is NOT necessarily a network issue. Those symbols pop up when your frames per second drop below the server tick rate. Official servers have a tick rate of 60 Hz, Portal servers have a tick rate of 30 Hz. This is why so many of you (myself included) never had issues in Portal servers, but have constant issues in the Official servers.
When your FPS drops below 60 FPS in the Official servers, you will see those three rectangles in the top left of your screen and will notice things being in slow motion, your gunfire not registering as you would expect, and general rubber-banding issues. Going off of a few other threads posted about this, I realized that my CPU was having to do a lot more work than it should because my RAM was severely underutilized. This would lead to frame drops from the 100s to the low 30s in large maps with 64 players, and then would lead to those time nudge issues where it felt like network lag. I generally play on Ultra settings with a few of the effects turned down to Medium.
My specs:
AMD 5950x
RTX 3080
32GB DDR4 RAM (4x8)
I realized that this whole time I've had XMP/DOCP Disabled in my BIOS, so my RAM was running at 2133 MHz rather than the 4000 MHz it was rated for. After updating my BIOS firmware to the most recent build from the manufacturer's website, I toyed around with DOCP until I eventually landed on 3600 MHz being the most stable for my PC.
I've played about 6 hours now in Official servers and have not seen the time nudge issue even once. So I think it's safe to say, at least in my case, this was an issue where my RAM was severely underutilized, leading to my CPU needing to make up for the additional memory requirements leading to rough and inconsistent gameplay. I would highly recommend that you guys check your RAM settings and see what Speed it is set to. (Shift+Ctrl+Esc on Windows 11, then click Performance, and then Memory -- you will see a "Speed" section, mine said 2133 MT/s, after Enabling DOCP and setting it to 3600 MHz it now says 3600 MT/s).
I want to give a huge thank you to @Oskool_007 for being so active in these threads. I figured out this was the issue for me because of their comments, thank you!