In my point of view soldering a row of wires to the MAV-NUS is more difficult than assemble the OSSC. But this is just my point of view!!!
For soldering the OSSC components you need the ability to solder large QFP chips with pin pitch of 0.5mm and some other small ICs with 0.65mm pitch. Also two of the QFP have ground / thermal pads on the bottom side (see assembly description: https://www.niksula.hut.fi/~mhiienka/ossc/diy-v1.5/assembly_tips.txt). I have soldered them using soldering paste and hot air.
If I were you I would look for some defective hardware with a few QFP chips. Desolder them and solder them into place back again. That’s good for practicing. Then you will see if you are able to solder them correctly 🙂