Thank you all for you help, tbh I'm even more confused now. I basically want to build a PC that will last and I don't really have to do much to, as long as it streams well.
Yeah, it is confusing and there are always priorities to balance.
When you say you want it to last, how long are you thinking of? And when you say you don't want to do much to it, does that mean not fiddling with settings, or not changing hardware? Would you be adverse to upgrading CPU in a year or two? Fitting an after market cooler later on?
Some considerations:
RAM - Ryzen does benefit from faster RAM, and the actual benefits vary from task to task, but have diminishing returns after 3000-3200MHz. Samsung b-die works the best on Ryzen and is the most likely to run at stated speeds selecting the standard profile with no problems. Some people have had trouble with Hynix RAM, but it seems perfectly reasonable to expect to run a Hynix kit at decent enough speeds as well, as others have pointed out.
I may have overstated the need for b-die, so I'm sorry if that was confusing. I wouldn't have have bought anything else, but of course that doesn't mean no one else would.
If you're shopping around this is something I'd look at when comparing RAM kits, but you'll need to balance it against budget as well. If you get a Hynix kit running at 2933/3000, there will be minimal difference to a kit running at 3200MHz. If you're definitely not interested in overclocking it's even less of a concern. I haven't overclocked my RAM yet, but I definitely want to see at some point if I can get it to 3400-3600MHz or thereabouts. That would be a nice boost, but it you're just leaving everything at stock, there will be less difference.
Case - yes, it is true that the case doesn't
really affect performance. Again, though, you need to balance things. Do you think you might want to fit another cooler sometime and try to get better performance? If you're sure you won't and you're not bothered about aesthetics, then no worries. But if you think you might, then it's worth checking what can fit and thinking about airflow. Also consider how bothered you are about noise and check reviews of cases for noise levels.
Oh, double check what length graphics cards will fit as well before buying!
Motherboard - if you're sure you're only going to go with stock settings and you won't upgrade your CPU, then the one you originally linked will be okay.
You'll be more likely to get the most out of future CPUs with a better board, but if you're not into swapping out your CPU anyway and you're definitely not ever going to overclock then it's probably not worth the money.
So maybe I was protecting my own concerns when building a bit.
I always think about what I may want to do in the future with a build and spec accordingly.
Will a 2600x with the other cheaper kit last six or seven years for high end gaming and streaming? Maybe. Hard to say, but it's possible. Would a slightly more expensive system last that long if you were to upgrade the CPU, install better cooling at some point, perhaps decide to overclock in the future? Again, hard to say but more likely I'd imagine.
It seems your budget isn't that flexible, though, so I guess you cut your cloth accordingly.