I had neglected to take the number of encounters per day into account. Working around that or using the spells recharge after X number of encounters approach might help. To be honest I can't think of any time I designed an individual adventure based on number of encounters; I usually just do what make sense in narrative. Also, what kind of session length do you do for 6-8 encounters per day? It seems like you would have to track spell slots between sessions or do 8-12 hour marathon games. We are lucky if we get in 4 hours for a session anymore (family obligations and such) and a lot that is spent on general hanging out and tomfoolery.
i run typically 3 hour sessions weekly and yes absolutely the players are required - as am i - to record things between sessions for the next session. it is just simply unrealistic to have sessions always end in a long rest everybody back to full.
then again, we have always had that, ever since 1e and beyond. its nothing new. AFAIK 1e did not have any "if session stops mid-event you all get everything back" rules yet it did have spells per day, uses per day and so on.
All 5e has done is really shifted things away from "per day" to the "rests" between them.
But a key thing to keep in mind as GM is not that 6-8 is magical. its an example of what the system was tuned around.
The real key is that you should often enough create a series of encounters that challenge the party and are spearated by short rests and about half as often that are separated by long rests.
it could be as simple as "big fight - short rest - big fight -short rest - bigger fight - long rest... three enouncters one per session over 3 weeks with a little bit of non-challenge in between.
but the main thing as Gm is to recognize that the challenge for a party right after a long rest fully charged and who can afford to just go full out - need to be higher than what the basic CR would produce because that Cr is based on wearing down.
Consider this - 5th warlocks get two spells per short rest at third level slots- which means six castings over the Sr-Sr-LR encounters. A similar wizard has 4-3-2 which means more spells overall but only two at third - spread throughout the three encounters. but... the sorc can dump all 9 spells in one encounter if they want.
So if you run games where the sorc and wiz are staring at three encounters split by short rests before a long rest OR a game where its mostly one encounter then long rest - those will play very differently.
yes i know both classes have other stuff so the balance is not limited to just this aspect, but it illustrates the point.
In my game, sometimes it is ONE BIG and out, sometimes it is the two-shorts to a long, sometimes it is many shorts befor a long, sometimes the big bad fight is first and then they have extremely tired and wounded but still have objectives against lesser opposition etc etc etc... the key in my experience is mixing it up **with intent in design** and to be very aware of the PCs involved and working to see balance in play.
power/value/balance is at the intersection of NEED and CAPABILITY so you know their capabilities and so if you want balance you have to provide the right needs.