I came up with an interesting idea for an encounter for my players to fight at some point in the future but would like some input on some of the details I'm not too sure on.

The encounter: This is a list of the spells and the cards they are attached to for reference.

https://docs.google.com/spreadsheets/d/19ctWQT4lZJyhSOsTM815KQaewvuksVXEMFHV6IjjCes/edit?usp=sharing

I made a spellcaster that uses a deck of cards as his spellcasting focus, and the spells themselves. Each suit is a different kind of spell type. Hearts are healing spells. Spades are AOE spells. Clubs are single target offensive spells. And diamonds are buff/debuff spells. The higher the card in the suit, the higher level the spell is. For example: A 2 of spades is a simple Acid Splash Cantrip, where the Ace of Spades is Cone of Cold. I intend on having the caster be limited to what spells they can cast by having a physical deck of cards to draw from. If i do not have the Ace of Spades in my hand currently, the caster cannot cast Cone of Cold. I also want to have the concentration for each of the spells be tied to the cards themselves. So, one of the spells I have is Flaming Sphere. I intend to make a point to the character that the card is floating above the sphere to hopefully clue them in to possibly being able to attack the cards to quickly end spell effects. I also have it limited to only being able to use 1 card of each suit per turn. Basically getting 2 spells on their turn, and 2 legendary actions to use the other two cards. I have yet to come up with stats for the spellcaster, or anything more past the spell list.

Depending on when I throw this at them, it will either be 6 level 5 or 6 characters. My characters are an echo knight with boots of speed and the mobile feat, a circle of stars druid with a staff of healing, a wild magic barbarian with a grey bag of tricks, the sentinel feat and a pet griffon, a paladin with a Periapt of Wound Closure, a rogue with a Homebrewed dagger and what i will simplify as the Omnitrix(which doesn't get used much), and a ranger npc that is joining them for their current mission. They all have at least a +1 weapon and some other magical items. So, at any given point, there could be 10 to 11 targets on the field. They have as much as they do because it is all but one person's first time playing DND and want them to have cool things, so they have some options and it is not just "swing sword, end turn." I did introduce these items slowly as they got more comfortable with their characters.

One of the big questions I have is how big should my hand be? I'm starting with 8 and drawing back up to 8 cards on the casters turn. Is that too much? Not enough? I'm not sure. Does anything here seem unbalanced? Anything I have thrown at these characters has been a relative push over for them for a while.

Any input is appreciated because this is my first time DMing and want to give my players creative but challenging encounters to play.

Suggestions:

Have a way for them to discard any cards they don't want at the end of their turn. Lots of these are pretty bad spells, and if they can be ended by simply destroying the card they won't last long depending on how hard it is to destroy a card. A hand of 8 cards has a petty good chance of having a card of each suit in it every round if they can refresh as many cards as they like. 

Honestly, I'd also get rid of the whole "healing" suit entirely, enemies healing is a waste of time mostly, it just drags out the fight it doesn't make it harder. Plus you desperately need more chance of getting AoE damage spells if it is likely this enemy will be fighting 7+ party members. There's what? 5 spells capable of dealing damage to multiple targets in the entire deck? There's such a low chance for the baddie to get one of them...

You also need this enemy to have friends as well, buff spells are only of value if you have allies to cast them on. Now if this caster has a Troll ally that it can cast Haste, and Fire Shield on that's a bit scary, but also those spells are so unlikely to come up... 

I agree that you should get rid of the healing suit. It would be better if that suit was necrotic-esque spells. 

I'm not necessarily concerned with the discarding of cards, since all but a couple of the spells are not usable at any point. I have dimension door and transportation via plants as an escape if he can manage to. But other than that, all the cards can be used. I also decided to get rid of all but 2 of the healing spells. The ones I left were Aura of Vitality and Healing Spirit. I only left those two in because they are healing over time and can be targeted to be destroyed. I was planning on having the cards have a relatively high AC, mostly to reflect the size of the target. Something around 20 was my initial idea. 

Since I removed the healing spells, I replaced them with some of the lower end damaging 5th level AOEs, some 4th level AOEs and Summon Greater Demon, and lastly a few more 3rd level AOE spells. 

I was planning on just having the buffs target the caster, but some allies would be fine especially with some of the spells that target several allies.

https://docs.google.com/spreadsheets/d/1rNwnN81CZp8faMkE_tcV1N6RvUOLZ8wxDghimGSqUQE/edit?usp=sharing

You should make a whole class based on this. Like you would get new cards as you level up and would try to make the best deck possible. I would really like to see that as a class, I think it would be fun.

Can I use the spreadsheet to help make a class based of this?