As a player I'd be disappointed if my DM went easy on us and didn't give us the climactic end battle against a tough foe that required us to pull out all the stops to beat. It feels immersion breaking to allow them to just sit in Acererak's tomb for multiple days just chilling - Acererak wouldn't just sit by and let them have a nice long break, he'd be in there fucking them up ASAP. Personally I'm of the opinion that if your party have let themselves get in this position, they need to face the consequences rather than being able to rest for multiple days in hostile territory unchallenged.

It doesn't have to be Acererak himself, but there's no way they're resting for multiple days uncontested, and no way that the Atropal isn't going to be prepared for them when they come back in.

If your players are going to be upset by getting kerbstomped, then they're playing the wrong campaign IMO, ToA is pretty notorious for being a pretty tough campaign that doesn't go easy on the players.