My players are finding out my BBEG too quickly

Hi, posting on mobile, so forgive my formatting.

I'm a relatively new DM. I've DM'd a few games, so I would say I'm probably like a level 4. I'm currently doing a homebrew campaign where the players (8 of them) are trying to settle down in this town in the Underdark that was founded on anti-slavery principles, founded by this escaped slave named Elkantar. I've built him up to be a folk hero, like a really good guy who's charismatic and likeable and most importantly, very handsome.

The twist is that Elkantar isn't an escaped slave at all. He's actually the oldest son of the Xorlarrin House, and he was furious when he was passed over for leadership despite the fact he's a once-in-several-generations wizard. So he's left his family and started this town to gather blood sacrifices for a sealed dracolich god named Ratri so that he can use her power to take over the entire underdark. It's all a really cool build up.

They're still in early game, but a few of my players have grown suspicious of him. A few nat 20s have shown that he carries a necromatic object (the phylactery), he panics when people threaten to leave the town, ect. ect and now I'm just worried he's gonna blow his cover well before it's time, narratively.

My players are really thinking outside of the box and rolling well, and I want to reward them for doing so, but I feel cheap by the way I'm hiding it. He magically made another necromatic object that is more innocently explained to throw them off his scent of a phylactery, he always has a new lie for every one they catch him in, ect.

One of my players has started a romance with him, and it's going to be so juicy once this player figures out his true intentions. But I want the build up! If they find him out at level 4 he's just going to kill all of them and that's not fun at all!

What would you do in my shoes? Do you think that I'm being cheap in the way I keep inventing things narratively to hide it? I mean, I'm even considering instituting a rule that a Nat 20 isn't an automatic success because they nearly found his ass out last time! Should I let him be outed? They still don't know about the sealed dracolich god...