Display lag or something else?

[Erling1]

0.) TestMem5 (1 hour), OCCT (1 hour) — with no errors.

Insufficient time frame to come to the conclusion that it's stable.
12 – 16 hours is the minimum time you should spend on evaluating IMC stability.
RamTestPro, TM5 with different configs, GSAT, Ycruncher & others. Each should be tested for a minimum of 12 – 16 hours.

1.) Attached screenshots
3.) RAM is running on stock XMP with slightly tweaked subtimings. CPU has PBO enabled with Curve Optimizer set to -10, everything else is stock.

Paste the FrameView data in a pastebin and send the link here. My assumption is that you're entirely CPU bottlenecked, as Valorant isn't that GPU intensive. I see that you're pushing 400-600 FPS, which would push you in the "Nvidia FastSync is a viable strategy" crowd.
Chasing tCL is irrelevant for performance in esports games.
Don't use Curve Optimizer at all. It leads to more issues than it solves.
Keep PBO to the default of your motherboard
I suggest that you evaluate whether XMP is even stable to begin with before chasing tighter manual primary/secondary/tertiary memory timings.

2.) I have 3 internet providers (TENET, Westelecom, and a third one I stopped paying for as I saw no point in it). My ping to Stockholm and Frankfurt servers is 40–45 ms. According to the graph, my ping fluctuates slightly—something like 40–42–44–45 ms for about a minute, then it jumps to 52 ms and drops back to 40–45 ms. My Bufferbloat score is A or A+. Packet loss during a match is usually 0, or at most up to 5 packets (0% percentage-wise), everything is always fine.

Bufferbloat generally doesn't matter for networking quality.
You could check which ISP is best in your region by conducting a MTR to a specific server that Valorant offers or using BGP tools.
This is just a "nice to know" troubleshooting step though.

I'll try doing what you suggested with the RAM.
As for the internet, I honestly don't know-I've tried a lot of things. Even though it shows 0% packet loss, the feeling in the game is awful.
Result: https://pastebin.com/pwjN0R7a
And I'll turn on Nvidia FastSync as well