For the PSU, most single GPU builds won't be using more than 400W at stock. The unit you're using right now is on sale for a good price though ($110) so it's a "why not", but if you do happen to purchase a PSU when it's not on sale, I wouldn't discount the 450-550W units unless you:
1. plan on going HEDT down the road but want to keep your PSU
2. plan on getting a flagship CPU + GPU and want to apply heavy overclocks
3. have some insider information saying future CPUs and or GPUs will be more power hungry by a significant margin
Even a 3950X + Radeon VII setup pulls ~500W worst case at stock going by Anand's 3950X review and TPU's GPU power consumption charts.
You probably won't be using a Radeon VII but it's the most power hungry card of this generation so everything else will have lower power draw. There's a good chance you'll be considering an NVIDIA GPU if your CAD program can take advantage of CUDA.
Current PSU designs are most efficient at the 40-60% load mark. If you really care about the efficiency for some reason and will be running your system under load 24x7, then a PSU with capacity equal to double your maximum load would be preferred. In the real world where your system is sitting idle most of the time, however, it'll be operating in a lower efficiency range so in the end it really doesn't matter.
As for the CPU cooler, you can go with a Scythe Fuma 2, Scythe Ninja 5, Thermalright MACHO, or Thermalright TRUE Spirit 140 (careful on the height for this one) instead unless you really need the compactness of the NH-U12S. They're better performers and cost less. I'd also say the NH-U14S but I assume you went with the U12S Chromax just to avoid the brown fans (no Chromax version of the U14S).
The MSi B450 Tomahawk MAX is considered the second best B450 ATX board in terms of power delivery and can handle a 3900X or 3950X stock without any issues. The only "upgrade" would be the B450 Gaming Pro Carbon AC which uses the same power delivery circuitry except adds on doublers for more current handling capability. I would consider the B450 Gaming Pro Carbon AC if you really want Intel LAN over Realtek LAN or want the included Intel WLAN. If you were going to get a wi-fi card anyways, this is a really good deal actually since the cheapest Intel 9260 Wi-Fi cards with the M.2 to PCIe x1 adapter cost around $35 while the Gaming Pro Carbon AC only costs $15 more than the Tomahawk MAX. If you want to overclock a 3900X or 3950X, upgrade to one of the good $150-$165 X570 boards.
The MSi MAX boards come with Ryzen 3000 support out of the box although most B450 boards at this point in time should as well unless it's extremely old stock or their BIOS chips are too small and the vendor was too lazy to provide a BIOS with Ryzen 3000 support at the expense of Ryzen 1000 support or a less "pretty" BIOS UI. New stock of the B450 Gaming Pro Carbon should have support out of the box. If it doesn't, it has BIOS flashback (i.e. you can flash a BIOS without a CPU installed).
3600MHz RAM is a good choice for Zen 2 CPUs. Unlike other CPU architectures where we just meme memory overclocking, Zen CPUs have their Infinity Fabric (the die interconnect) clock tied to the memory clock so it's actually worth trying to max out this interconnect's clockspeed. The Zen 2 CPUs (Ryzen 3000) will usually have their infinity fabric clock max out at 1900MHz (3800MHz memory) on chips with a golden memory controller. Almost every chip should be able to handle 1800MHz (3600MHz memory).
Micron E-die kits are known to clock pretty high very consistently but their timings will usually be slightly looser than Samsung B-die kits. The 3000 or 3200 versions of the kit I added use the same ICs and both hover around $140-$155. Sometimes the 3000 kit is cheaper, sometimes the 3200 is cheaper, sometimes the grey or red or white color version is cheaper. It's a well documented kit and you can always just use the Ryzen DRAM calculator to get timings instead of having to manually experiment.
You could go B-die but last I checked, even the cheap good 2x16GB B-die kits cost $200+. The differences in timings are going to be unnoticeable unless you're constantly pushing memory heavy workloads where the extra 1-3% of memory bandwidth might be useful. Supposedly B-die is also a bit harder on Zen's memory controller than E-die (for reasons I don't know, but buildzoid and many other overclockers are saying the same thing) so it might be harder to get your desired speeds with B-die (3600-3800 isn't that high though so it shouldn't make a difference).
Also there isn't any difference between "Ryzen Ready" and "Intel Ready" RAM. RAM is RAM. The interface is an industry standard and it won't vary by platform. There might be some kits where the XMP profile has speed + timing combinations that work fine on Intel CPUs but not on Zen due to differences in the memory controllers. With those kits, you'll have to manually tune them to get better performance over the basic JEDEC settings.
For the SSD, I just choose whatever is cheapest among the Phison E12 + Toshiba BICS3/4 64/96L TLC drives at the time. There's a thread in the SSD subforum that has them all listed. The only difference between them is the amount of space they reserve for overprovisioning.