The required planets for this are:
Eos
Voeld
Havarl
Kadara
Elaaden
Getting 100% is pretty easy and does not require all side missions to be completed. Just getting the vault, outpost and forward stations will get you to 70+%. Then killing the architect (Havarl doesn't have one) should get you over 80, a few side missions later and you're done.
Special Note: On Elaaden, choosing to keep the drive core from the Krogen and not having an outpost will still net you 10% in viability, making you still able to 100%. However, you will lose access to the architect side mission.