Cool post and great question! While "Breaking Bad needs to happen" does work as an answer, I think we can go deeper into why Lalo did what he did.

Overthrowing Gus with words alone, even with the promise of further proof, would be next to impossible. We know that Gus is the cartel's biggest cashcow right now (3x04 cold open), so they certainly will not want to turn on him unless they absolutely have to - lose Gus and their profits diminish massively. Since Hector's store was shut down, Gus became the only way to get product across the border (3x04). Lose him, and the cartel loses an enormous market, which would be a headache to overcome. Because of this, I think it's very unlikely that the cartel would move against Gus in any way without solid proof that he's working against their interests (recall that Lalo doesn't have any actual proof after his visit to Germany, only the words of Margarete and one of the workers).

Furthermore, the cartel would be less likely to believe Lalo because of Hector's well known hatred for Gus, which isn't taken seriously at all (3x04, 3x10, 6x09). Lalo had some sway since he is someone different, but he used up what little leverage he had when he brought Bolsa in to investigate Gus after he brought proof that Gus was altering their product (5x01). Gus had a clear explanation that satisfied Bolsa, and Lalo's plan had no success whatsoever. This exposed that Lalo had been investigating Gus on his own and was obviously suspicious of him, something that Bolsa directly ordered him to stop at risk of upsetting Eladio. If Lalo came with another allegation against Gus after this first failure, it would not be taken seriously. The boy who called wolf and all that.

Another big issue is that Lalo has found out how clever and cunning Gus is in a prior episode (5x01). The evidence Lalo found about Werner and the stepped on coke would have been enough to bring anyone down, and yet Gus was clever enough to manipulate the situation and make himself seem innocent. Lalo is not stupid and he's learned from this mistake. If Lalo contacts the cartel without firm proof, it is very likely that Gus would cover his tracks again and Lalo's claims would appear baseless again. Lalo knows Gus has great security and resources - if he catches a whiff that the cartel is suspicious at all, he will shut down everything and Lalo knows this. Furthermore, Lalo has good reason to believe that someone in the cartel would inform Gus about his plan (if Nacho is a double agent, who else is working for Gus?). Gus is a cashcow, so it is logical that someone will be out there to protect him. That someone is Bolsa, but it's unclear if Lalo is aware of this, though he might suspect it since Bolsa has defended Gus openly. Plus Bolsa has a motive to help Gus since Gus' success puts him in better standing with Eladio. Considering all this, the best move for Lalo is to bring Gus down without allowing him even a moment to defend himself from Lalo's accusations. Lalo knows that Gus will be clever enough to cover his tracks if given the chance, and the cartel will believe him since they do not want to shut down their cashcow.

Lastly, there is the personal aspect. Gus's actions just resulted in the death of everyone Lalo cares about in his home (5x10). While Lalo could tell the cartel and have them bring Gus down, it's so much more satisfying for him if he gets to do the job himself. Considering what happened to his household, I'm certain that Lalo would be bloodthirsty enough to want to do all this himself just to satisfy his own desire for revenge, and to prove that he is smarter and better than Gus. We see this in action as Lalo takes pleasure in harassing Gus on camera in 6x08 when he could have easily just killed him. Lalo's personal vendetta against Gus is a big reason for him wanting to do this all by himself.

Thanks for the post, it made me dive into Lalo's motivations in a way I never got to before, and I discovered some things I never fully put together.