Any "quick fix/all-in-one" fitness or nutrition solutions. While there are minute optimizations for elite athletes, 99.99% of the population can adhere to the general consensus of nutrition and health science.
Do something that gets your heart rate up for at least 30 minutes a day. Speed walk, bike, row, shoot hoops, jump rope, doesn't matter, just get your heart pumping hard for at least half an hour a day.
Roughly a third of your food should be fresh leafy greens & veggies. A third should be whole grains and unprocessed starches and sugars like sweet potato and fresh fruit. The final third should be a protein. Lean meat like fish or chicken, or if you're veg/vegan, beans, tofu, seeds, peas, etc.
To build strength, general bodyweight exercises combined with stretching is fine for most people. If you wanna get really strong, get a few kettle bells or adjustable dumbells on the used market for $50-$100. You don't need an expensive fitness club membership or one of those all-in-one $2,000+ fancy machines that mounts on your wall.
Don't drink often, don't smoke, don't pound stimulants like caffeine or nicotine.
Brush your teeth well.
Get 6-8 hours a night of good quality sleep.
Keep your brain engaged, read, play music, play games, learn a language, etc.
I'm speaking from experience, because I have fallen for stuff over the years that promised fast results and optimal methods with minimal effort. Fact is, unless you're training for the Olympics or you have very specific heath conditions, those basic bullet points will cover the vast majority if general health and fitness.
I agree with almost everything you said, except I wouldn't advocate for people including stretching as a regular part of exercise. Despite what people tend to think, there isn't really evidence to support broad general benefits of stretching. Obviously, if you are a gymnast or another type of athlete with specific needs for range of motion beyond what is "normal", go for it. It may not hurt, but it is likely a waste of time, and if you are constrained in the amount of time you can spend on exercise, you should spend that time doing things with well established benefits, like weightlifting.
The other thing I want to add on (again cause I agree with what you said) to the diet part is that people probably shouldn't trust products like Athletic Greens to "count" as their daily vegetables, despite their marketing. I haven't been able to find good research on it that wasn't funded by them. Also, just more generally, I'm skeptical of the purported benefits of juice and smoothies. Again, it's hard to find good studies on it, but much of the benefit of fruit and veggies is in the fiber and resulting delayed digestion, so it stands to reason that the processing removes some of the benefit.
In almost all cases, it's cheaper to have a gym membership at a decent hardcore gym.
There are a lot of things you simply can't do with bodyweight alone. And you can't do it with just a couple kettlebells and adjustable dumbbells either. Having a lot of strength and muscle mass when you're young is a very strong predictor of health in old age, since past the age of about 40, people just start losing mass and strength; the more you have before that, the better off you are.
I said $2,000+ to encompass even more expensive machines/setups.
I never said bodyweight or a kettlebell set could provide exercises for every possible movement or strength vector.
I said that the vast majority of people don't need anything more than those to build a healthy level of fitness. And given that the average cost of a gym membership in the US is around $50 per month, after a few months, their used kettle bells or simple dumbell set has already paid for itself.
And weights last basically forever unless they are severely damaged, so zero maintenance cost.
Nothing wrong with going more hardcore if that's your thing, but that's not at all necessary to build a solid base of strength and general fitness.
Body weight exercises can build plenty of muscle. You only need specialized muscle targeting once you're body building. For health body weight exercises are ideal, targeting individual muscles is not as useful to fitness as training many muscles in tandem for common movements.
If you want to get really strong, you might want protein and creatine supplements to speed up your progress, but even that's not necessary and they only speed things up a little.