Tasha is in the game already.
its save at turn start, unfortunately (guessing bug), and I don't think you can start with it as a wizard during character creation...maybe I missed it, but I didn't see a way to get it like a goolock or anything.
[Regarding sleeping] Yes but you can sleep right afterwards and recharge everything, have another encounter, go nova again, then sleep and so on, that way your caster doesn't need to manage resources at all. As a DM I can tell you that resource management and recomended number of encounters per day are basically the of the main ways used to ballance caster classes compared to martial classes.
It was more that the resource drain in during the fights, not that by the third one you are drained. We will target the gobbo fight, specifically, because its tedious and annoying to restart if you end up having to. Parties aren't meant to do with 2 encounters at once, it isn't a linear scale of "well if we add twice the people and 1 encounter takes half, the double should use all". The enemy DPR kicks up and wipes you out, you just cant mitigate it with a certain headcount because of how many people spells at that level affect.
I destroyed a ladder the grand total of once (in the Zhent hideout to be precise). The most bloated and unfun fights I had in BG3 were in the goblin camp after I killed the 3 leaders and that's not because it was a difficult fight. It was actually pretty easy as I used my casters and archers to get rid of lower hp enemies (a lot of them with 12 hps so killing them in one hit with spells or sneak attack was not a problem), the problem was the time it took for the thing to end when the outcome was never really in doubt. As for your final lines... you are making a lot of assumptions about D&D. Yes, PCs are supposed to be powerful but depending on the group playstyle and level of experience they are not guaranteed to just walk around winning every fight, I for one don't DM the game that way although I also try to refrain from killing PCs due to sheer bad luck (on the other hand I'm the kind of DM who believes actions have consequences, if you do something utterly stupid in one of my games you are probably going to pay a price for it). If all I threw my players were easy encounters designed to make them feel great about themselves they would just ask me for more challenging encounters. And again the recomended number of encounters in D&D 5th edition is 6-8 encounters per day!
Edit: forgot to mention that currently wizards can learn any spell in the game, cleric spells included. This is wrong and needs to change imo but it certainly gives wizards a lot of utility they don't usually have at their disposal, especially not early on.
My party is level 8 in Avernus and they are going to walk into a room thats 50' high and 100' wide (circular). 2 Erinyes that can polymorph into giant spiders, 2 phase spiders, a shadow demon, and a fomorian. The place is littered in webs that will halve the speed and the room is hot and gives them a level of exhaustion every 2 rounds on a failed DC12 Con Save. I don't really run easy fights haha. The 2 chain devils riding wyverns and dropping them from 200 feet after they were taking crush damage was a rough one, just some random encounter. By powerful I mean a fully charged party focusing on one thing is powerful. I ran tomb of annihilation. 5 level 10s almost killed a CR23 Lich AFTER fighting something that was a DC19 con save to not because exhausted every round and that thing had 250 hp of its own. Lich had power word kill, maze, all sorts of nasty shit. The way the map is though, he cant really move around. Thats crazy though. It why all bosses have escape, legendary, or minions because players will annihilate solos. That is the issue though is for every head you put onto the initiative tracker, that power gets more and more and more diffused. It should be a battle of skills, spells, and being clever. At a certain point a large number of individually mediocre enemies A. Isn't fun B. removes utility from a lot of abilities and spells as it just doesn't accomplish much. 100 skeleton? You thunderwave and kill 10. 90 left. Cool spell?
This is up to DMs and players as to what they enjoy, of course, but I have found over the years that a spell landing or an ability sticking having a marked effect on the field is a lot more gratifying.than saying "Hey I got Goblin 34 in hold person for like 1 round until I got hit, im out of spells now". People like seeing that impact, like having the spell slot that this forum and others have already hammered on needing to be more important (resting mechanic), so if Im going to use a spell it should be friggin important, I dont have a lot of those. Resting afterwards takes away that decision making but, realistically, people using spells should still be a cool thing and its a lot more toned down than what I am used to seeing from 5e casters.