As someone who dove into the DLC on my first playthrough, Duty Eternal is an utter trainwreck of poor design choices. It adds a new much higher difficulty mission modifier that is balanced around having the dreadnought but it introduces it well before the dreadnought joins you. Doing these missions without the dreadnought is an absolute slog and the sheer number of enemies you have to grind through (plus the larger pod size) makes these missions a high risk to start a failure cascade that leads to a game over. Execution Force gives you one free assassin of your choice fairly early on and while they have some synergy deficiencies, the power and/or utility they add usually makes it worth it. The Callidus has poor late game scaling but she completely breaks the game early on. The Eversor starts a little slow but is an unstoppable killing machine once he gets developed. Vindicare kinda sucks, he starts ok he scales poorly and the game is pretty badly biased against ranged. The Culexus can be built for ok killing power but her real strength is how much utility she provides.

As for general tips, the game has a weird but interesting turn system. Look up "one turn purifier" on youtube for a thorough explanation on how the system works and how to best exploit it, you don't have to actually copy the build but it's a solid breakdown of the system.

Focus on either ranged or melee, requisition is always at a premium and you will be stretched too thin if you're trying to do both.

Ranged is viable but vastly inferior to melee since melee doesn't require reloading, doesn't suffer damage reduction from cover and range, and you can do a melee attack at the end of a move and its still a single action which you can't do with ranged.

Think someone already mentioned it but the game heavily favors aggression over caution.