So I'm homebrewing a creature that has the ability to summon things that buff its attacks and gives it resistances/immunities. The stat block is shown below.
Toadstool
Huge monstrosity, unaligned
Armor Class 17 (Natural Armor)
Hit Points 261 (18d12 + 144)
Speed 15 ft.
STR 24 (+7) DEX 10 (+0) CON 26 (+8) INT 3 (-4) WIS 10 (+0) CHA 7 (-2)
Saving Throws STR +14, CON +15
Skills Intimidation +17, Perception +14
Damage Immunities Poison
Condition Immunities Blinded, Deafened, Frightened, Poisoned
Senses Darkvision 60ft., Passive Perception 14
Languages --
Estimated CR 24
Fungal Body. The toadstool can’t be blinded, deafened, frightened, or poisoned, and any critical hit against it counts as a normal hit, unless it is incapacitated.
Buff. The toadstool becomes more powerful depending on the number of sporecaps summoned, as seen below.
1-4: The toadstool deals an extra 1d10 poison damage on all attacks, it can release an extra boomshroom.
5-7: The toadstool deals an extra 2d10 poison damage on attacks, it gains resistance to all damage from piercing, slashing, & bludgeoning damage from nonmagical attacks, and it can release an extra 2 boomshrooms.
8+: The toadstool deals an extra 3d10 poison damage on attacks, it gains immunity to all damage from piercing, slashing, & bludgeoning damage from nonmagical attacks, resistance to all damage from piercing, slashing, & bludgeoning damage from magical attacks, and it can release an extra 3 boomshrooms.
Actions
Multiattack. The toadstool makes two attacks, one with its bite and one other action.
Bite. Melee Weapon Attack: +7 to hit, reach 5 ft., 1 target. Hit: 23 (3d10+ 7) piercing damage.
Sporecap.(Recharge 4-6) The toadstool summons 1d4 sporecaps, which are large, tree-size mushrooms. The sporecaps buff the toadstool. The number of sporecaps determines how powerful the toadstool is. These sporecaps have 5 (1d10) hit points each.
Boomshroom. The toadstool releases 4 (1d8) Boomshrooms. Boomshrooms are small mushrooms that grow and explode, and any creature within 5 feet of it takes 12(2d12) poison damage.
Spore Bomb. The toadstool attaches a spore to a creature of its choice within 20 feet of it. This spore explodes at the end of the target's next turn, creating a 5-ft square of poisonous gas for 1 minute. Anyone who enters the space of the cloud takes 13 (3d8) poison damage for each round they spend their turn inside the gas.
Ground Pound. When at 40% of it's max HP or less, the toadstool can hop on the ground, and any creature within 25 feet of it must succeed on a DC 15 Dexterity saving throw, taking 27(3d12+7) bludgeoning damage on a failed save, or half as much on a successful one.
How would the buff ability effect challenge rating?