Not in this case. Goliath with Heavy Armour Mastery, Great Weapon Master, with level 3 Warlock spells and fighter levels is level 9 at best. At some point investing in defense is just wasting resources. Once you aren't worth attacking all those extra defenses you've put effort into will be wasted. That, and with Armour of Agathys you want to be attacked.
Hex is a waste of a spell slot and concentration past the first couple levels as it is a very weak 3rd+ level spell. Hunger of Hadar though and other AoE spells are great in the right circumstances. Definitely good to have one known.
Most Warlocks won't want to use AoA, so much so that it would also be a waste of a spell known. It can be good though and it scales well. Shadow Blade is a great complementary 3rd level spell though not a great 4th level one. I also like Blade Ward as a known Cantrip with Pact of the Tome. That amount of defense won't usually be worthwhile, but there are situations where it will be very good and one Cantrip is a small investment.
Keep in mind that 15 damage is 4d6+1. Getting that twice plus the benefit of the temp. HP and not using concentration would be a good use of a 3rd level slot. It is also reasonable that enemies wouldn't be aware of the freezing effect until it is too late.