They weren't required but yeah, side quest is needed in order to build mc's stats and team stats. Don't be afraid no month, day or time cycle mechanic that kills the character... But the duration of the boss battle will likely kill some of them as I stated above (my own game experience). It is highly recommended, that you kill Arihama (the Warlord) in short shots so no dying character will deducted from the roost of crew members.
Though R.I.P to my Hakkon... his sacrifice will never be in vain.😭😭