What do I lose by going Paladin 17 / Warlock 3


Background

I have a 9th Level Oath of Redemption Scourge Aasimar Paladin. My intention was to take levels 10, 11 and 12 into 3 Levels of Warlock and pick up the Celestial Patron with Pact of the Blade.

This character is the only divine member of the party. As such, he is the primary tank and primary support.

Question

I know that I’ll be losing the Paladin’s Aura buff, but beyond that I see most people suggest Paladin 18 / Warlock 2 or Paladin 16 / Warlock 4. Why is this? What am I losing by only taking 3 levels of Warlock instead of 2 or 4?

Primarily I’m valuing the Paladin’s 5th level spell list as the reason to only take exactly 3 levels in Warlock.