People grouse way too much about the 6-8 encounter adventuring day with 2-3 short rests, IMO. Yes, that is the assumption around which different resource recovery systems are balanced. Yes, inter-class balance is at its best when you stick broadly to this guideline. No, the game will not break if you don’t follow it precisely.
Personally, I plan my adventures around this guideline, but I don’t enforce the sequence, and I allow my players the freedom to break it. I plan around 4-6 encounters per session, and I roll for random complications (which can include encounters) in dangerous areas. The players are free to take rests when they want, but taking the time to do so creates a risk of such random complications occurring. We generally get close to the 6 encounters with two short rests per adventuring day, but sometimes we get fewer, and that’s fine. It’s very rare that we get more, because the PCs are pretty worn down by or before the end of that time. It works fine.
I feel like with a lot of the game balance assumptions of 5e, people have a tendency to either worry way too much about adhering to them, or decide that they’re oppressively restrictive and actively avoid them while decrying them as terrible game design. I think both positions are far too extreme. Treat them like simple guidelines and don’t stress about following them to the letter, and the game will work out fine.