Trying to make a spell with a nat 20 bonus. Can it be done?

So I'm coming up with a Final Fantasy summoner-esque character for my own personal use. With that, I'm making a slew of homebrew spells to fit this character. I'm using Final Fantasy V as a template for all the spells I'm gonna be cooking up. But right now, the one that's giving me the most trouble is the first one, and that's Summon Chocobo.

I've got it something like this; a sight-based instantaneous conjuration with V and S components that'll only apply to Wizard that's done as an action (this'll be the template for all of the summoning spells going forward). I have it so that it does 2d4+3 bludgeoning damage (which is the Choco Kick). Simple enough, right?

But, I wanna do a critical success bonus where the Chocobo would be substituted for a Fat Chocobo if you roll a nat 20 on spellcasting. Then it does 8d6 damage in a 20-foot sphere radius from the chonky guy coming down from the sky.

Now, I don't know if any of this is legal or not. This is my first time doing a homebrew spell. I've been messing with the creator to make it that this perk can actually happen in D&D Beyond. I put on two modifiers, one for both outcomes, but on the character sheet, it only displays the 2d4+3 damage. I don't know what I'm doing wrong, and I don't know if this is even right to do in the first place, but I want to know if this is possible. I want to know if I can hardcode the critical success bit to the dice roller and the character sheet and all that.

This is a dope idea. Big fan of FFV.

Unfortunately, I don't think there's a way to do what you're looking for with a spell. Typically, spells with variable damage effects (such as Sorcerous Burst or Chromatic Orb) rely on the same type of dice, so the dice roller never has to contend with different dice options. While you can code the Fat Chocobo damage into the spell, I can't see any way to make the spell 'select' that specific damage roll.

With that in mind, I think your only option is to include the Fat Chocobo effect in the spell description and track it manually.

Eh, fair. I was already thinking of that when I was drafting up the Sylph summon. Basically deal 2d4+2 necrotic damage then have that heal any allies within 5 feet using the same value. Just do that shit manually because the system only has so many cogs.

Meanwhile, I realize that doing the next level of summons is just really simple. The Ifrit, Shiva, and Ramuh summons would basically just be Fireball, Sleet Storm, and Lightning Bolt but done via conjuration. I was kinda expecting that going in when I talked with some oomf's on Discord over how to handle the summon spells in the first place.