Sticking to the theme of things I've changed my mind about, nutrition is one of them. I was a big believer in the following statement, if it can't be sustained, it is not a real solution. It is not healthy.
A respected coach posted something about how humans are supposed to be an undulating organism. To say this one way is how you eat for the rest of your life is silly. It made me rethink and admit my thinking was a little short sighted.
Lets take biking as an example. People that bike enjoy biking. They may bike everyday. Then they decide to enter a 200 mile gravel race like the Dirty Kanza. They are going to have to do some serious specialized training. When the race is done, they won't sustain the volume and intensity of biking they needed to prepare for the race. It is also doubtful that they will say, well I did the race, I'm done biking. They are still going to ride. A few months pass and they decide to do a mountain bike time trial. A 60 min all out redline zone 5 race. It's going to require much different preparation then the 200 mile race. They train for that race and when its done they aren't going to keep the same training even though they will still ride their bike.
This is an analogy for nutrition. While there are some basics, it's going to come down to personal goals as well. If you are trying to lose weight, attacking that goal for 3 months and then taking some time off (a set time) and not being as strict. You are still going to eat well, (your still riding your bike) but not as intensely. Then go at it again with renewed discipline but also with a metabolism that isn't ground to a halt.
Taking it a little deeper into the periodized analogy. Humans that are healthiest have the greatest variability in their physiology. The HRV is a score of your heart rate variability. It is the healthiest when it's the most variable. Most peoples joints are healthiest when they have the greatest motion that they can control. Varying your caloric needs, varying your macro nutrition all can be healthy and help break through psychological barriers that may come from feeling you are depriving yourself of something.
My thinking is that do the basics well consistently. Protein, veggies, water. Ride that bike. Then attack something aggressively. Most people lose weight if they cut all sugar/breads/dairy. I think most people can do this with a lot of discipline for like 4-6 weeks. Train hard for that crazy bike race. Then I'd back off to not limiting stuff and going back to making sure you got the basics covered. Go back to riding that bike. During that 4-6 weeks phase I'd have days of low calories and day of higher calories.
Variability in the human has been shown to be healthy, I'm starting to think nutrition is no different. When you want a change, attack it aggressively and then return to moderation. Keep doing this.