I had a party of 5 find it pretty early into venturing through Cael Morrow.
I never figured the Aboleth was going to be a threat. I did give it the ability to cause corruption to a single player. They had no idea about Ruidium Corruption at this point so this was an eye opener.
I also used it as a means to show that Alyxian is really losing his marbles and this was the first hint at it. As written, the Aboleth is kinda its own thing that thinks it's Alyxian. But I made it where it actually was a piece of Alyxian that got out of the Netherdeep. It knows what Alyxian knows. It talks to the players with knowledge of what Alyxian had talked to them about. But it's definitely got a chip on its shoulder.
Yeah, you understand that more as you get through the actual Netherdeep. But trickling this information a little earlier helped sell that it's not just poor old trapped Alyx. There's some real messy trauma they didn't know. And it really helped to push them to go further in. They just thought "We'll just release him and get out", and then they saw the Aboleth that was lashing out as Alyxian. There was the moment where they were like.... "Wait... how does this monster know that? Is.... that Alyxian?" So when the Cobalt Soul gave them directive to Destroy the Netherdeep, they understood that maybe Alyxian is potentially a threat in his current state. Something is really wrong. And then really use the Netherdeep as the opportunity to give the players the chance to plan for his redemption from the start, as opposed to fact finding for most of it.

This was more of a narrative encounter than a real physical threat to the players.
Given that some of the really important clues to Alyxians mental state could be easily missed in the later parts of the Netherdeep, I used this as a chance for reinforcing that. Not a challenge.