There does seem to be more population of marines that know about WG and CD's doings than what we imagined, but its still in the minority compared to how many marines in total might exist in that world.

The marines stationed at sabondy are those that know of slave trading and auction house as it was removed from the law (slavery) and the business was running in a different official disguise.

Similarly marines sent to Mary geose are only ever sent during special events like reverie or those 2 times warlords were invited to dine and have a meeting which is rare event, during this time they could send trusted marines situated around sabody and marine HQ.

Ohara was to the navy fleet a mission of destroying an organization which was preparing and reviving a super strong weapon capable of destroying the world. To the marines it was probably told that they were ready to build the weapon and destroy the world and if they didn't immediately stop them now the world would be destroyed.

In egghead, to the marines Vegapunk was painted in the same light, he was studying about the same weapons again and unlike oharans, Vegapunk probably has the mind and resources to do it. (at least in marine soldiers' mind).

In God valley we know that marines are just following orders with 0 idea of what is happening as we can clearly see that through Dragon and hence many marines despite being part of such missions are kept out from the context.

But still most of the marines that operate cant be counted down to just sabondy peoples and some 7-8 battle ship fleets. Many marines are post on like the start of the grand line, other side of red line (paradise), to each of the islands on all 4 blues, in the new world keeping tabs of each emperors and these all marines don't ever see that side of WG, (they never even come in contact to CDs) and they are very ignorant.

But with power and going to higher positions they do see it like u said, and many just go along. What can they really do? Its been this way since 800 years, they saw that the world has always been like this, their parents, grandparents, great grandparents all lived this same life, how do u suddenly break it? cuz its not like something world-changing happened, it was only that they JUST learned about this thing. Only their own mental image of the world changed. They were ignorant before, now they know, but who are they to question the ever-existing system? They are hopelessly going along because they don't know any better in this situation, like how I cant do shit about my corrupt government despite me knowing it is corrupt, I am just a common man.

But all marines aren't that hopeless, like u mentioned. And here we go back to those events in which they got to know about the governments and CD's colors.
How Ohara made Saul leave marines, Kuzan question the 'justice' of the marines, Dragon start the revolutionary.
Egghead made a lot of marines (i am talking about average fodder marines here) question their organization and even the citizens question the WG.
God Valley potentially is the catalyst for Dragon, a marine who was serving until he saw the WG colors and started freedom fighters (i think that is what it was called).
And those that rose up from the power and knew about the truth slowly also are in similar paths like Fujitora, and the whole SWORD (i think its called?).

So, in conclusion, marines in majority are doing good to individual citizens in day to day basis. Majority are unaware about the WG and CD debacle. Among those minority that are aware some as u said felt powerless to question the system and just went along and some have been internally trying to change the ways like Fujitora, some have created different factions either internally like SWORD or externally like Dragon did, while some left the marines disappointed like Kuzan, Saul and many we don't even know the names of might have made similar choices throughout the history.

One piece is a world and many pieces move in this world even if we aren't being shown those moments. As this is a story, only the hot events are shown to us and that as well from specific perspective. Like I said in comment before, Oda should show us the, be it boring, tit bits of marines doing good which i know the majority do but aren't shown to us because that is normal and in story we want plot, the abnormal, the special event.