I'm not looking to bog down the flow of play with these, just make every decision more consequential.
I think you're missing some factors...
Hard Mode
Spell save DCs are 6+CasterSTATmod+Slot Level.
Casting a spell requires Concentration and provokes an opportunity attack.
Wizards Clerics & Druids prep spells directly into slots.
Rituals still consume a slot, but it can be a slot with a different spell prepped into it, and performing a ritual does not require concentration.
Legendary Mode
In addition to the above.
Using a cantrip costs a slot, but you can use the cantrip at-will until you recover the slot. Cantrips that scale work at the level at which you first gained the slot used.
While Concentrating, all attacks have advantage on you, you have disadvantage on all rolls not related to the spell being cast or maintained.
Spells have a casting time equal to their level. They are completed on your initiative count less casting time. This means a readied attack can interrupt a non-cantrip spell, and enemies close to you in initiative might move or attack you while you're casting.
Slots at each spell level are halved for all casters.