The stats seem right-ish.

Damage from pillar is 17.5 + 16.5 = 34. Damage from press is (8.5 + 5.5) * 2 = 28. Total damage is 62.

Your monster has AC 15, HP 210, attack bonus +8, and damage 62.

The DMG guidelines say that a CR 12 monster should have AC 17, HP 240, attack bonus +8, damage 77.

But monster manual on a business card, which reflects stats closer to what monsters actually use, says that a CR 12 monster should have AC 15, HP 180, attack bonus +8, damage 60.

This is pretty close.

Whether the resistances are appropriate will depends on your party.

Is your eighth-level group running around with nonmagical weapons? That would be a bit unusual for a group at that level. If so, you should use a monster with fewer hit points, since all your characters' attacks will deal half damage.

If your group has magic weapons (or relies on spell attacks, in the case of the druid), then the resistance to nonmagical weapons doesn't matter.

The other resistances and immunities are fine, and are appropriate for an ooze-type monster.

CR12 seems appropriate

I went to Kobold Fight Club and told it you had four level-eight characters and they were fighting a CR12 monster. It told me the encounter was just on the edge of "Deadly", which is probably what you were shooting for.

Think more about the poison

Both of your monster's attacks say "the target must succeed on a hard save or become poisoned".

How long does the target remained poisoned? One round? One minute? Until they short rest? Forever?

Being poisoned is pretty bad because it gives disadvantage to all attacks. With your monster's AC 15, this will make it quite difficult to hit. My advice is to make the poison have a short duration, like "until the end of their next turn", so that this isn't crippling for your group's melee attackers.

The stats seem right-ish.

Damage from pillar is 17.5 + 16.5 = 34. Damage from press is (8.5 + 5.5) * 2 = 28. Total damage is 62.

Your monster has AC 15, HP 210, attack bonus +8, and damage 62.

The DMG guidelines say that a CR 12 monster should have AC 17, HP 240, attack bonus +8, damage 77.

But monster manual on a business card, which reflects stats closer to what monsters actually use, says that a CR 12 monster should have AC 15, HP 180, attack bonus +8, damage 60.

This is pretty close.

Whether the resistances are appropriate will depend on your party.

Is your eighth-level group running around with nonmagical weapons? That would be a bit unusual for a group at that level. If so, you should use a monster with fewer hit points, since all your characters' attacks will deal half damage.

If your group has magic weapons (or relies on spell attacks, in the case of the druid), then the resistance to nonmagical weapons doesn't matter.

The other resistances and immunities are fine, and are appropriate for an ooze-type monster.

CR12 seems appropriate

I went to Kobold Fight Club and told it you had four level-eight characters and they were fighting a CR12 monster. It told me the encounter was just on the edge of "Deadly", which is probably what you were shooting for.

Think more about the poison

Both of your monster's attacks say "the target must succeed on a hard save or become poisoned".

How long does the target remained poisoned? One round? One minute? Until they short rest? Forever?

Being poisoned is pretty bad because it gives disadvantage to all attacks. With your monster's AC 15, this will make it quite difficult to hit. My advice is to make the poison have a short duration, like "until the end of their next turn", so that this isn't crippling for your group's melee attackers.