I would suggest against this.
It’s your first time DMing and this session is one “that will introduce 3 players to the game”, they won’t want to have to deal with an extra set of rules that will add a deck in the flow of combat.
It’s a bad idea as a DM because:
You haven’t run a combat encounter yet, keeping track of player HPs, creature HP, creature strategy, if they’re new players, they’ll be level 1, which can be easy to kill, you’ll be explaining and re-explaining rules to new players too, for the combat mechanics, for each of the class mechanics. You’ll have to also explain each tarokka power, why the tarokka deck is affecting the characters, what the tarokka deck is. You might draw a power that ends an encounter or kills a player, making it less fun for everyone.
For the nat1/nat20. Keeping track of something else between rounds is another thing to deal with, when you’ll already be juggling a lot, and it’s your first time. There is already a rule for crits that make them exciting. Use that.
You’re asking because you can’t foresee how this will effect how you run combat.
Build a baseline with standard rules before you add anything.
This is bad for players because:
Depending on what your players want, they might want to RP, in which case adding complexity to the combat is going to put them off. They might want to just kick-ass as adventurers, in which case adding complexity to the combat is going to put them off.
Don’t underestimate how much players, especially new players, will be looking up when they start combat, even level 1 characters. If you’ve seen a lot of playing on streams it can look easy, because they’ve prepped a lot or are used to improv and making a decision and going with it. New players will have a lot of confusion and fear about “doing it right” or “not interrupting the fun with questions”, when learning together and finding your own game is part of the fun.
Why I think you’re really asking:
You’re excited and nervous about DMing, you want to do well and you want to make the game special and unique for you and your friends.
Here’s the secret, you will. You don’t even have to try.
Use the base rules before you add anything, get used to them, be comfortable running combat as DM and spend the extra brain space making sure all your players are comfortable and having fun. Get used to how your players play, what they’re interested in, then add using that.
The secret sauce is in how you all play together so you don’t need to add these rules, at least not for a while. There’s plenty of fun to be had with simple rules and the simpler the rules, the easy the choices for you and your players and the more those choices will shine.