Resource management and you
What this ultimately comes down is what type of game you and your table want to play. A big part of D&D is resource management and the encounter system is a major contributor to the expenditure of those resources.
How you want to manage and challenge your party is very much going to be up to you. But that doesn't mean that this is opinion-based as there are many factors and options to consider.
Time is a limiting factor
The first thing to consider when planning is how much time you have. You've stated that your sessions are 6-8 hours and that you'd like opportunity for both roleplay and combat.
My own experience has shown that for parties of 6, you can usually get 2-3 extended combats and a roleplay opportunity within that time. The larger the group, the more time combat will likely take as you have each players turns to consider and the time they'll take.
Time may also be a consideration in-game as well. If you're only able to do 2-4 total encounters per session, it may take 2-3 sessions (or more) in order to be able to have a long rest and refresh the resources. If your players understand and are on board with that, then you can work with that - but it does mean they'll have to wait in order to get that long rest.
Encounter volume and difficulty
One thing I've noticed is that "easier" encounters tend to not tax resources at all. You are effectively trading session-time for an encounter that may not drain any actual resources. That's not a bad thing, but it's something to consider. If the point of encounters is resource management, but they're avoiding the use of resources because the encounter was simple, then you've mostly just lost time in session for something that hasn't made an impact on their resource planning.
This doesn't mean you should go the other way and do the 15 minute adventuring day and have a single very difficult and extended battle, but it's something I've considered in my planning.
It's also important to consider that the recommended 6-8 isn't limited to combat encounters, but encounters in general. Chapter 3 of the DMG covers a lot of this and lists sample encounters that cover both RP, Combat, and a mix.
Challenge and on the fly planning
What I tend to do is build fights that are very difficult according to Kobold Fight Club, and see how they go with the players. C0nsider the in-game time and see how things pan out with the battles. You can add combat to the adventuring day or postpone them based on your players resources.
Playing your monsters
It's also important to consider the intelligence, planning, and in-combat response that your monsters will have. Intelligent monsters played intelligently are going to be very difficult and present more problems for your players to solve. Don't be afraid to swing hard - you can always back off or present/accept opportunities for your players to do creative things.
For rests, I do follow the 1 Long Rest/day requirement. If they use a lot of resources first thing in the morning, then that's something they'll need to deal with throughout the day.
For Short Rests, I'll allow their use as long as it makes sense in-game. Wanting to rest after every fight could be a thing, but that's why there are generally other in-game things going on that suggest expediency is a thing so that they don't dawdle. Taking that hour could happen, but then they may miss out on completing the larger mission, meeting an important NPC, etc.
My planning style
I generally plan for about 2-3 very difficult combats per adventuring day along with 1-2 roleplay opportunities. This seems to allow for challenging fights and fun roleplay within a session and not make the players feel like they're waiting forever for a resource refresh - or that the fights are so simple to not require resources and just an easy slog.
I often have an encounter scheme set up and monsters I can use, but then will adjust depending on how it's going to keep it fun for everyone. Too easy isn't fun, and TPKs usually aren't fun, either.
I also try and make it more than just Kill Everyone. Resolutions outside combat can be an option, 'games' within the combat (prevent the door from closing, hold the passage until reinforcements arrive, etc.), and environmental considerations can also make the combat more interesting. You can read up some interesting cinematic combat ideas here.