Could still do better on the mobo and PSU. Is there a budget?
The cheapest cooler is the stock cooler. Unless you're going to overclock hard or want a particularly silent build it'll do. If you do want either of those the 212 Evo and cheaping out on the cooler in general are questionable.
Nothing obviously wrong.
NH-D15 might be overkill.
Not sure which part of the bait-and-switch-cycle of "good CX, bad CX" that particular CXM is.
CPU or GPU encoding for the streaming?
Haven't looked at the monitor yet. Hard to tell if it's what you want beyond being a 2k 144Hz monitor when that's the only requirement I've got to work with.
None of that is worth the price/hype if you're already happy with the performance you're getting. There is no improvement to be had. You're upgrading for the sake of upgrading, just to get rid of 3000$.
Everything is only getting faster/cheaper so why would you upgrade before you need to?
Long text. Looks interesting/complicated. Might require thinking. Sleep now. Thinking tomorrow.
Yeah, upgrading to dual channel should be fairly cheap even if it doesn't do that much and most of those games should be fine even with 8GB, so I'd do it if it's still within budget.
SSD seems unecessary, nice for loading times but not worth the effort, especially considering this isn't going to be a long term situation. If you do have one lying around or can afford it you can just use it as a second drive for the games. Should still speed up loading and it's not like that PC will be rebooted that often so shortening the wait for something that happens once a day isn't a priority. No need to reinstall windows.
GPU seems reasonable.
An aftermarket/factory overclocked 580 can easily draw over 200W so I'd say that's a hard no. It might technically be less than 300W total but with spikes, +5V rail and the cheap 300W OEM PSUs being what they are I wouldn't expect it to cope well with that.
1650S with just 100W TBP should actually be doable and most of them should be a bit short as well, so higher chance of finding one that actually fits inside the case. Check the PSU first though. If it's one of those terrible ancient leftovers with just 150W on the +12V rail and >100W on the +5V rail it's not going to work. Also check if it even got a PCIe 6-pin power connector.