Caloric deficit vs Metabolism issues

The amount of deficit you should be shooting for will depend on how much weight you have to lose. Someone with a lot to lose could safely have a deficit of 1,000 calories a day. Someone who is losing a few vanity pounds is going to want more like 250 calories a day. There is no right answer for everyone.

Things that "boost" metabolism are really common sense. Basically, more movement. This is another reason not to have too steep of a deficit. If you're not consuming enough food, your body is going to try to protect you by making you want to move less, which further reduces the number of calories you're using each day.

Adaptation, explained by a doctor:

(from the "Most Helpful posts" here on MFP)

https://community.myfitnesspal.com/en/discussion/1077746/starvation-mode-adaptive-thermogenesis-and-weight-loss/p1

There's really no such thing as starvation mode or wrecking your metabolism with calorie cutting. Yes, our rate of thermogenesis changes somewhat but not much and not as something we can control by simple adjustment of calories. Effort is effort and requires calories to make it happen. If you reduce your efforts because you feel lousy because you aren't eating enough, your calories burned will go down.

That is not to say we know everything about nutrition and calorie expenditures. We know enough, however, to know that reducing calories-in means you have fewer to put out or, if you continue to use them, must convert your body's store of energy into calories. This works up to and including actual starvation-caused death when the body has no more stored energy available.

The sweet spot you seek for reducing calories-in is where you feel good enough to maintain your use of calories. It's a matter of health. You need sufficient nutrients associated with those calories to keep your present body healthy while you reduce the size of that body. Crash, very low calorie diets often fail because the person doing them is made, by their diet, to feel lousy and move less. They may lose a good deal of weight but cannot maintain that lifestyle indefinitely.

Those of us who wish to be a healthy weight with a healthy body and to maintain it must learn to eat and exercise in a way that maintains that. We can never go back to our old ways without reattaining our old weight. It is a long process requiring a change of mind, assumptions, and habits.