111Kur

If it were me I would try a few things.
 
1.) Remove AIO completely, keep plugged in to SATA power and powering the pump at full tilt I would literally orient that AIO in every direction possible to get all the bubbles out.  Bubbles in block = heat.  You will hear liquid sloshing around even in NEW AIO's but if you do this and believe its missing ALOT of liquid (and you do this and it doesn't work for dropping temps) I would do step 2
 
2.) Buy a cheap reservoir, cut a line from block to AIO radiator and drain.  This is a mixed metal AIO (alum and copper, like 90% of the AIOs on the market) so you add the reservoir (barb fittings) to the loop (custom loop!) and fill with antifreeze (I use Prestone 50/50 premix with their patened Cor-Guard - corrosion inhibitor - and antifreeze is a natural biocide so no need to add anything else) to replace liquid.  (you can google people fixing AIOs this way, and I personally run all mixed metal custom loops and use antifreeze as science beat this long ago - corrosion issues - plus the AIO companies use something similar anyways).
 
If none of that works (res with barb fittings is like $4-6 on ebay depending on the day) then I would say the 240mm radiator isn't going to cut it.  (not enough surface area for dissipation so the liquid is to hot so it loses its ability to absorb the heat efficiently off the coldplate - copper block - )

I think you will find the D15 a great air cooler.  I myself want to buy one just to compare it to my custom loop with 2x240 and 1x120 mm rads in push pull config.  I paid about $100 for my loop build (china ftw) ordering parts and a D15 with fans is about the same.

That sounds like the kind of stuff I'd be happy to try on my second system.. I'm not too happy with having "open" liquid in this one, though, or having any more downtime than necessary. I think I'll buy a noctua d15 and call it a day. From what I heard, it should give comparable temps to the corsair AIOs? Some people even talk about better ones.

Next step - You have an air bubble trapped in the CPU block.  Turn pump on full blast and let it run for a while, then pick up the machine and orient it in every direction possible sans the block ever being above the radiator or highest point in the loop to get that air bubble out of the block.

i use google chrome at like 50-60 degrees and my cpu is fine. to my knowledge i have never gone over 85 even under a cinebench load. you could try raising your fan speeds. mine are adjusted to ramp up faster.

Best place it can be for an AIO.  However doesn't mean its not trapped, which is common.  Since its a V1 I would expect you bought this used or have had it for a while so it may even have more space for air after some slight evaporation in the loop over years.  
 
Put pump on 100%, let it run a while.  This may push the air bubble out itself.  However I have the most, and quickest success, when I run the pump and then pick up the PC and literally go through a sphere of movements without ever putting the block at the highest point and use gravity to assist in removing the bubble

Make sure the pump is working by listening to it or feeling it. 
Re seat the cooler. Re apply thermal paste.
Only use hwinfo64 or/ and ryzen master to monitor ryzen temps correctly.

Are you using Corsair's software for CPU temp monitoring too? If so, use Ryzen Master or HWiNFO64 instead, as those are the most accurate for Ryzen systems.
Also, I'd really advise you do a fresh install of Windows if you haven't already.

My experience with my H100i is that the backplate and mounting screws (screws have extra piece on it that rotates freely) are a problem with correct mounting.  Reach behind the block between the board and the block and see if any of the 4 mounting screws have any wiggle room in the "free moving" portion of the set screw - if it moves its not properly secured, if its rock solid to board then the standoff is on correctly.

for the love of god stop running the system under load when you see temps of 90+ and remount the block to ensure proper seating.
Make sure as well you took the protective tape off the bottom of the pump if its new, and or that the thermal paste isnt dried out if you didnt reapply the stock paste. That very much sounds like either a bad mount, bad pump, or bad paste. Since you crossed the pump off pretty well then move on to the other two.

take off your cooler, wipe your cpu and aio with a cleaning rag, then reseat your aio and make sure it is on tight enough. from my experiance with an overclocked r9 3900x these temps are like 10-20 degrees too hot. or, it might be because of summer heat. when it is hot in my home, i experience similar temps

As I said 
Re seat the cooler and re apply thermal paste and make sure the pump is working not by checking icue but by listening to it
Is the bios up to date?  

Odd my AM4 bracket is the 4 holes, GTX is the V1 version I thought (I have GTX) - guess I was wrong!

X570 chipset has more features than B550. Looking at the X570 version, it dropped the 2.5G Ethernet. The X570 version has more PCIe expansion slots with more lanes, better audio solution (with optical S/PDIF) and more USB3.2 Gen1 instead of USB2.0. But you didn't list any of this as things that you look for. What you looked for was decent Ethernet and B550-A PRO wins there because everything else you cared about is the same on these boards.
 
There is an argument to be made that you can upgrade Ethernet with a PCIe NIC but the 10G cards today are still over a 100$, 2.5G NIC can be had around 30$ though. The other thing is that most of the consumer network gear still don't support more than gigabit Ethernet. So if you need to upgrade later, the prices for faster NICs and network gear and other devices are probably going to come down.
 
So, even though the B550A-PRO has better Ethernet, I would probably get X570-A PRO because it has more features and you can get better Ethernet with an expansion card later. If you really care about faster Ethernet and can use it today, go with B550-A PRO.

Thanks guys! That's the one I'll be going with, then. 

B450 tomahawk would be the cheapest I would go, but if you're not overclocking it's perfectly adequate. However I would recommend trying to get the MSI B550 tomahawk or Mortar instead as they are better all round and have better future support. The mortar should be little cheaper. B550 would definitely be the platform I would recommend with a €150 budget. However, there aren't many reviews yet so I would try and wait a few weeks. Hardware Unboxed on YT should do a roundup of the best board sometime soon, and that should give you an idea of the best choices at your price point

I would go with B550 - you will get support for at least next generation of Ryzen. I found three boards under 150€ at Amazon.de (which is the only German seller I can look at, I'm not from Germany). Then I compared the boards and found out that the clear feature winner is this one:
 
MSI B550-A PRO - 149€
This is the only board with 2.5Gb/s Ethernet. Since you are working with media, this might come handy down the line. Per your requirements, it's ATX, has 4 RAM slots (you will not get more with Ryzen AFAIK), one USB Type C, internal Type C header (for front panel). It has one more 10Gb USB A port - not Type C but same speed.
 
Boards with B550 are more expensive because they have more features. You can go with B450 which is cheaper but depending on the motherboard, you won't be able to upgrade your CPU. Also, 6 SATA ports might be harder to come by since B450 supports less SATA ports than B550.

Searched a bit on Alternate, they got lots of B550 boards, got the same board like @redteam4ever.
Here is the link: https://m.alternate.de/MSI/B550-A-PRO-Mainboard/html/product/1647937