Would love to hear more about how that is going Caliban !
How do you find your success in that role ? Are you finding it fun ? What levels are you all and what are the other party members builds ? How do they work with you / synergize with you in that role ? What do you find are the most effective strategies and what do you find doesn't work so well ? Was it your intention to be in the that role and did you build accordingly with that in mind from the start or did you end up in that role for some reason ?
Overall it works pretty well, but it requires you to play a lot of attention and micromanage your abilities for best effect. For my preferred build, the idea is to get out there and absorb damage. Dealing damage is a lower priority - you are there to block the way to your main damage dealers and not die.
I've played 5 or 6 "tank role" characters with various levels of warlock over the last three years. It's hard to get away from that first level of Fighter for Heavy Armor proficiency, Con save proficiency, Fighting Style, and Second Wind.
Most of them have been Fighter 1/Warlock X (Fiend patron/Blade Pact), with a high Str/Con and only a 14 Charisma. Use Armor of Agathys combined with Blade Ward and Heavy Armor Master at low levels to preserve the Temp HP of the Armor of Agathys as long as possible. By the time those run out you are hopefully able to reduce something to 0 HP and gain more Temp HP. Eventually pick up Great Weapon Master so that you do enough damage that you can't be ignored. Only a few spells are devoted to damage, most are utility/defense - Hex, Misty Step, Darkness, Dispel Magic, Counterspell, Fly, Dimension Door, and eventually Wall of Fire (with Repelling Blast instead of Agonizing Blast so you can knock enemies into the wall). I do take Eldritch Blast, but it is mainly for when they can't get into melee with their opponent (kind of a magical replacement for Javelins).
I prefer Blade Pact for flavor reasons (and the second attack), but it can work just as well with other pacts, especially if you go Fighter or Paladin 5 for the second attack.
But one caveat - most of the games I've played these characters in haven't gone past level 10. My highest level one is an Adventure League character who is Fighter 8(Eldritch Knight)/Warlock 7 (Fiend/Pact of Chain) with a Legendary Greatsword and Belt of Fire Giant Str.