I just can't see why Putin would have him killed. It makes no sense to me, he was already in prison, why make him a martyr now, before elections? As an idea elections in Russia are this month.
We also know Tucker interviewed him shortly before Navalny died and we know that Navalny chose to be in Russia. Maybe CIA wanted to not allow people to process the interview and move to martyr Navalny. There are some spicy stuff in the interview like the US blowing Northstream pipeline and Nazis in Ukraine so who knows.
Every media and politician is 100% sure Putin killed him without even considering any alternative.
My take is that the CIA killed Navalny in order to start unrest before elections or to make sure normies keep funding Ukraine.
If the UK had launched into a war with russia after any of these killings, you might have something. But they didn't. They didn't really capitalize on any of them. It's just not plausible to kill so many people over decades to "pin it on russia" without ever doing the pinning. At this point they'd just be working for russia.
Also the sheer number of conspirators you'd need for polonium, novochok, bombs, gunmen, heart attack laser satellites or whatever you think killed Navalny, this would need official sanction by higher ups.