You'll want that SSD as soon as possible. Saving your CAD 3D work constantly on HDD will be extremely annoying.
Also if you work with a lot of 3D objects, it might be more worth it to get a Ryzen 3 3300X (or even 3100), and get some entry level GPU like the AMD RX 550.
Especially since the 3400G is a Zen+ processor even though it's named like a Zen2, while the 3300X and 3100 are true Zen2 processors with better single-threaded performance. So the 3400G is not a recommended processor for this use case. It's good for general usage, but you're doing more than that.
So I would overall build something like this:
-A320M motherboard with NVMe M.2 slot (there are many variations of the Prime A320M, I don't know which you're going for)
-NVMe 120GB or 240GB SSD. Important: there are NVMe (PCIe) and SATA M.2 SSDs as well, they go in the same slot. An NVMe SSD like the Kingston A2000 series is barely any more expensive than a normal SATA SSD, but has 4x the speed. You'll definitely feel it. This is why you need a motherboard with NVMe M.2 support.
-8GB RAM is OK for now, but know that Ryzen CPUs benefit a lot from high-speed dual-channel memory configurations, so you'll want to get that 2x8 as soon as you can if you want to get optimal performance. 2666MHz is the minimum, something around 3000MHz would be even better. Ryzen CPUs do make a difference out of that.
You didn't list all the other parts (power suppy, case, hdd), but any good quality 400W PSU would be enough for this build even if you choose to get the RX 550.