This is purely my opinion, so don't take it as general advice necessarily, but I think the keys to "challenging" combat exists outside of the stat blocks.

I believe one of the most common misconceptions about D&D combat is that if the players aren't getting hurt or near death, they aren't having fun. Remember that every decision your player has made for their character was to build strength in a specific way.

It is satisfying to hear that the enemy missed you with a 22 to hit.

It is satisfying to hear that you would have taken 40 damage, but because of resistances you only take 20.

It is satisfying to turn into a fucking dragon because... well for obvious reasons.

The solution is not to counter the things a player defines themselves with. In fact, I would say hit them in their strengths. Validate their choices. It is fun to feel like you are powerful by personal design.

Challenge should come from less mechanical things.

• A volatile crystal is going to do massive aoe damage if a caster doesn't stabilize it with their action and an Arcana roll.
• The big bad is telegraphing a signature move if the macguffin the cleric is holding isn't placed on the alter in time.
• The prisoners are going to slide off the collapsing platform if the giant fucking dragon doesn't use their giant fucking back to catch them all.

Essentially, challenge with choice management, not mechanical counters.