191x7

If you've been using a 3400G for so long, just add more RAM and upgrade to a 5700X. You'll be amazed how huge of an upgrade it will make. Then save up some more money and upgrade to a next-generation AM5 platform with a Ryzen 9700X or similar.

I would not upgrade to a Ryzen 5900X, not on that X370 board with weak VRM-s. It's the warmest and most power-hungry AM4 CPU. In your place, I would either swap to a 5700X, a 5700X3D/5800X3D, or upgrade to a completely new platform. The 5900X is, at this time, a failed purchase since the 5950X is stronger for work while the 5700X, 5800X, 5700X3D and 5800X3D are way better for gaming and such tasks. With a 5700X you get double the cores of a 3400G, the cores are a lot stronger and it supports faster RAM. It's a huge upgrade.

Did you peel the sticker off the cooler base?

Your temperatures should be lower with a cooler like that. How old is your thermal paste? Is the cooler clean? Are the fans in push-pull? Tried reapplying the thermal paste? Latest motherboard Bios? Tried a negative PBO offset of 15 to 20?

AMD also has AntiLag and AntiLag+ but limiting the framerate without having a CPU bottleneck causing stutters is still the best option.

You can try turning XMP back on and downclocking the RAM to 3000 or 2933. Having the RAM on stock JEDEC 2133/2400 limits your Ryzen a lot.

There are two things you should try. 1st - disable the fast startup / hybrid sleep in Windows (or just disable Hibernation) 2nd - try to locate a vBios update for your GPU.

Update your Bios to the latest. Retest. Then get those: -a Ryzen 5700X -a cpu cooler better than the one that comes with the 2600

Go and read my comment again.

Yes you should be fine. The P12 Max do not have the PST so you can't chain them without using a fan splitter/adapter.

Might be a long shot but... Have you installed the drivers for your monitors? Are your GPU drivers up to date? Checked the other drivers with something like Iobit Driver Booster? I mentioned the RAM frequency because of the RAM clocks the Zen+ architecture supports. I was going to recommend Latencymon but you already beat me to it. Does the same happen when one of the displays is connected to the motherboard (using the GPU integrated in the CPU)? Do you have the APU drivers installed too?

3.3V is brand new, 3V is standard. 2.8V or lower means dead.

There are Arctic P fans with "PST" which allows you to use multiple fans on a single header. For example, the ones in the picture use 0.1A of current meaning a 1A motherboard header could, in theory, run 10 of those. But 0.1A sounds too low to me, most 120mm fans are 0.25A to 0.35A so I would not put more than 3 on a single 1A header. But some motherboards have 2A headers.

Arctic Cooling P12, P12 Max, P14 and P14 Max.

Check your CMOS battery voltage with a multimeter.

You can try disabling hardware acceleration in your browser. The chipset drivers, do you download those from AMD or the board manufacturer? Use the ones from AMD. Are you using a high-refresh monitor? Is your refresh rate set to the max your monitor supports? Adjusted the delays in the Keyoard settings in the Control Panel? Agesa 1207 is the first agesa that properly fixes the USB stutters and some other issues. You should aim to use the latest Bios. Since you're on a Zen+ architecture I would recommend setting XMP (DOCP) on and then lowering the RAM frequency from 3000 to 2933 or 2666. But, more than that, you should aim to upgrade your CPU, the 3400G is a huge bottleneck to a 6800XT, and CPU-s like a 5700X or 5700X3D aren't that expensive.

You can try disabling the GPU Ultra Low Power State ULPS (by using a regedit or MSI Afterburner) and the MultiPlane Overlay MPO. But I doubt the issue is caused by a low GPU clock, it would be a first. No matter the clocks, the GPU refreshes the image, and the monitor refreshes by the refresh rate. Are you sure it's not a USB device issue? Latest chipset drivers installed?

Which exact RTX 3060? Latest vBios on it? Resizable BAR enabled in the Bios? DirectX updated using DxWebSetup?

Just headphones aren't the solution. And there is no "single best". If you want the lowest latency, using the onboard audio chip (usually Realtek and that means a modern Realtek like the 1200 series or newer) is recommended. Suppose you want the best audio experience (but a bit higher latency) with perhaps hard-to-drive headphones/IEM-s (high impedance or low sound-to-noise ratio or both). In that case, a decent DAC AMP unit or a stack (separate AMP and DAC one-on the other) is recommended, and eventually a good dongle DAC with inbuilt AMP. Then you have to decide if you want open-back or closed-back headphones. or IEMs. or both to interchange. In-ear monitors (IEM-s) sit between open and closed-back headphones when it comes to sound, but they do not cause "gamer hair" since there's no part that would sit on top of the head and push your hair down. Soundwise, you want gear that doesn't limit you as the player and lets you hear all the stuff you're supposed to hear. And that's for headphones - Sennheiser HD 560S, HD 490 Pro, Fiio JT1, Beyerdynamic DT 990, DT 900 Pro X, 1990, HarmonycDyne Zeus For IEMs models like - QKZ x HBB, Truthear Nova, Moondrop Lan, Truthear Crinacle Zero, Simgot EM6L or better. Check Fresh Reviews for the reviews and comparisons, he does for Apex, Valorant, CoD, and RS6. As for the AMP&DAC, I'd say you'd be fine with an Akliam PD4, Truthear Shio, Moondrop Dawn Pro, or something similar.

Disable the MultiPlane Overlay MPO. Retry.

Some games have an option to limit the frame rates, this is the best way. You can use MSI Afterburner and Rivatuner to limit your frame rates overall. AMD has the option too in the driver software, but if you set it global and have a game profile without the limit the game will not be limited unless you either edit the profile or delete the profile.

Please check the exact PSU wattage you have since your main post shows a 550W Corsair PSU. If it's a 650W PSU for most GPU-s you can afford with the 600 GBP you have enough and there's no need for a new PSU. Let me make another recommendation, instead of a GPU and the 5800X3D get a 5700X, a GPU, and a 1440p 144Hz+ monitor. You'll have a better time. How are the monitor prices in your area, any decent 1440p 27" to 32" screens for 200-300?

Have you checked the CMOS battery voltage?

I hear fan noise and people talking. What's wrong about it?

Going with a 5700X or even the 5700X3D would probably work fine with that board. The 5900X is the problem. Maybe you're still inside the return window? The 5900X is better for productivity, but it's bad for gaming, comparable to a Ryzen 5600 in most cases.