There are numerous people in Daenerys' council who know secret ways in and out of the Red Keep.
Varys: Back in Season 1 Varys is shown to know the secret way in and out of the Red Keep when he shows Illyrio in and Arya sees them. He also uses secret tunnels and entrances for Tyrion's escape.
Tyrion: Tyrion escaped through secret tunnels and has used the secret entrance himself when meeting Jaime in Season 7 Episode 5.
Davos: Davos smuggled Tyrion to the Red Keep to meet Jaime and also places a boat there for Jaime an Cersei's escape plan in Season 8 Episode 5. He is also an expert smuggler so presumably knows a few other ways in and out.
As such it seems obvious that they could have smuggled a handful of soldiers in to the Red Keep to try and assassinate Cersei and avoid the need for a battle altogether.
Whilst Davos may not have suggested it to Dany he could have told Jon who again may have dismissed it as not honourable but again might have used the plan. And Davos is probably against the whole sneaky assassin thing anyway after the shadow baby. However, it seems right up Varys' alley to want to do this and Tyrion wants to avoid fire and blood in King's Landing all together so they would probably have suggested it.
And even if that strategy isn't viable Arya and Sandor simply walk into the Red Keep with Arya not even hiding the fact she is armed. They could have dressed a few soldiers in rags and had them blend in, in the same manner.
Dany herself probably isn't against it as they sneak the Unsullied into Casterly Rock when they take it in Season 7. And she's not "mad" at this point and probably still doesn't want to rule ashes so would at least think it through rather than outright rejecting the idea.
Why did they not suggest this as a viable attack strategy?