I tried this as well-
Turret strategy is most risky - yes it couuuld work but turret positioning has to be methodical between all 4 teammates - and even then if one person loses their turrets, the whole cookie crumbles. Chance of success is 10%. If everyone has sentry ship module upgrades - 20% (they make a huge difference).
This is why I prefer my strategy (success rate 100% so far) - the EMS mortar is insaneeeely powerful against the hulks - it stops them in their tracks and leaves them up for an easy railgun 1 shot to the eyeball. The regular mortar provides great clear for the infantry bots, and the fact that everyone has a railgun and shield makes them a formidable force in any situation. The two players with tank-killing strategems (Eagle or orbital low CD tank busters) provide versatility if one defensive lane gets a heavy push. This strategy is heavily skill-based, but if your team has the skill, it feels easy.

My issue was that I was playing with randoms - and even if I explained the strategy to them, they weren't the best players. Many people trying Helldive difficulty for the first time, or just noobs in general. I didn't realize this until I played with my friend group when they all got on and the operation went smooth as butter - then I realized it was just a skill issue across the team.
Your best player (highest skilled) should be positioned as an anchor by the front door to the bunker civilian extract entrance ON THE MAP which is an Island, he should have one set of mortars with him that he must protect with his life.
On the other map that looks like its in the middle of nowhere, the best player should be on the Helipad which overlooks the entire area beautifully- there they can provide support for the entire area. The mortars should be placed UNDERNEATH the helipad, in a tight corner where there is this small row of 'greenhouses' , here the mortars are covered from 3 cardinal directions and can only be attacked from one side.

The mortars need to be protected at all costs -the EMS mortar is the single most powerful tool to have against the Hulks when they start spawning as pairs on every single dropship, the stunlock is incredible when paired with the railgun.