How can I reduce the number of encounters per day without throwing off game balance?

I’m running an Open Table sandbox game this summer. The 5E DMG states that they expect 6-8 Medium or Hard combat encounters per day, with about two short rests. If I tried to do that, we’d never get anywhere (short sessions, lots of newbies who struggle with combat), and my players and I enjoy many things besides combat, so I generally have closer to 2 combat encounters between long rests (though I usually make them at least Hard).

However, this means that PC’s rarely feel the need to take a short rest, and people can use spells and other once-per-long-rest abilities pretty freely, which unfairly advantages casters (who have limited big effects and unlimited but less effective cantrips) over mundane classes (who have more consistently medium damage output and defensive capability). It also means that the benefit of effects that recover after a short rest, like Warlock spell slots, are much diminished.

I’d like to restore the balance and at least have the option to make my players sweat a little about resources. But I do not want to throw combat after combat at them – none of us would enjoy that much in a row, nor do I want every combat encounter to be super deadly; I just don’t want them to start every fight at full strength.

The DMG suggests (under Adventuring Options: Rest) making a short rest 8 hours and a long rest 7 days. That seems promising, but a little excessive, so I’m considering making all rests 8 hours, but in order to get the benefit of a long rest you’d need to be in at least an outpost or otherwise reasonably safe and comfortable location, so the PCs can still have that moment of “I’m exhausted, let’s retreat” without needing to wait a whole week before continuing.

Has anyone tried this, or other methods to maintain game balance and challenge with fewer encounters per day? What were the results?