Some general responses:
The spell was Feeblemind. Solar's are immune to charm, but not enchantment. I rolled poorly against the effects.
I'm honestly not interested in running 6-8 encounters per day. I run 2-3 that are "hard" because I hoenstly don't want my game to be as follows:
You encounter kobolds
you encounter kobolds
you encounter kobolds
you encounter kobolds
you encounter kobolds
you encounter kobolds
you encounter kobolds
you encounter a Dragon.
I want to run few fights that are fun and memorable. I don't want to throw a million pieces of trash at the party. Not to mention, I honestly don't want to bother with running that many stat blocks. Lower levels, 5 goblins are a serious fight for 4 level 1-2 players, even tactical pwlayers and well-built characters. Early on in this same game, I killed party members when they fought a single group of 6 guards (reflavored Skeletons).
That's not really how it goes, but whatever. It's shouldn't be a major surprise that a tool doesn't work as well when you're not using it for its intended purpose.
To run fewer, harder combats and still keep CR roughly meaningful, you might want to look into rolling encounters together per the DMG's phased encounter guidelines - roll two encounters into one by having some sort of trigger for the second encounter within the first, have the enemies come in waves, etc.
A "harder" encounter in 5e isn't usually an encounter of a higher CR, it's an encounter with more creatures.
Beyond that, if CR is actually: APL/4 divided by 6-8 encounters then it is absurdly off base. Again, I had 3 party members. One spent the encounter sitting out because they disagreed with the other party members actions. One party member dealt with some guards while another party member (no magic items, brand new, fresh out of the box character actually) one-shot the Solar.
It's possible to one-shot a single encounter with a lucky roll. This allows for luck to be meaningful in the game's narrative. If you'd like to reduce the level of luck that can apply to the narrative, you might have to do some more creative things (give certain "powerful" creatures legendary resistance, for one).
I really shouldn't have to restrict players to the bare minimum in order for the game to work. That's a sign of something being broken. I never did this in 4e and I killed party members with beasties straight from the books (usually the later books yes but largely no tweaking required) from 1-18 (the highest we ever got). But this feels way too much like the 3E "we didn't really design this well, we know that, suck it up." attitude.
It's fully intentional, it's just not designing to the goal you're expecting. 5e can be swingy at the level of individual encounters. It is probably intended to be that way, since the dynamic, changing environment keeps things interesting. A lucky roll might beat the powerful monster. 5e in general isn't too worried about that happening. 4e was VERY worried about that happening, so of course it was less swingy.
If you want 5e to provide pacing closer to 4e, you'll need to take it upon yourself to a certain degree, because 5e's pacing and 4e's pacing are different, with different goals. 5e designs for the
day, 4e designs for the
encounter.
To redesign 5e for the encounter, some legwork might be required. Legendary Resistance, some sort of attack-based LR, upping monster HPs, having monsters come in waves, etc., can be some of the arrows in your quiver.