For some reason many sources stated you can only get 4 points per mission which is completely untrue.

Both original game and Enhanced edition will grant you 5 points per mission (which makes perfect sense since there are 5 bars to fill) if you;

1. Find all relics (guaranted)
2. Make at least 500 total kills (guaranted)
3. Make at least 150 special kills (might be more, this one is harder to test)
4. Have at least 3 psy gates by the end of a mission (guaranted)

And now the reason for everyone's confusion - a Mechanicus bar. Almost all bars lie and this one is totally broken and the requirements are super unclear. You definitely need to hack/destroy a number of turrets and doors but its almost impossible to tell the exact numbers. After some tests I've done the minimum seemed to be:
at least 6-8 interactions with doors (3-4 blocked and then destroyed doors) and at least 9 interactions with turrets (turning on, hacking and destroying seem to all count as 1 interaction each. So you can just turn on and hack the same turret 9 times and it will be enough). So:

5. At least 6-8 points in doors and at least 9 points in turrents for Mechanicus bar.
It might be that the game only counts a total number of points, say like 15 regardless of whether it's doors or turrets but it is nigh impossible to test it.

Anyways, it's the best I could come up with and I've been getting 5 fervour points each mission ever since I figured a broken mechanicus thing and the other minimum requirements. Everything else like filling a fervour bar, checking all rooms or finding all monitors matters not.

Hope you'll find it useful
Best of luck, brothers!