So, "Keto flu" is just a name given to describe the symptoms of electrolyte loss that often comes with the water loss and is 100% preventable. It sounds like you would be doing what's needed to prevent it, so assuming you're getting around 4000mg or so of sodium a day, every day, and enough water to satisfy thirst, you should be fine. The weakness may just be due to the transition from sugar burner to fat burner. Your body essentially has to build all your little fat burning factories so that you can use fat more effectively. I think that's something that continues to happen for a long time, but I guess the energy increase often comes a few weeks in. Seems different from person to person. What are your sources of fats? If you're not eating the right fats, you may not be able to get the energy you need from them. Basically, stay away from vegetable oils and margarine and anything low fat. Coconut oil, specifically MCT, can be a good fat for quick energy and is particularly ketogenic.
Another question would be, do you do any exercise? You may feel less energy if you're used to exercise already and are new to Keto. That might continue for a few weeks, but many just continue their normal habits and do what they can, until it gets better.
The only other thing I can think to consider is how much water do you drink? If you tend to force yourself to drink beyond thirst because you think getting a certain amount of water is good, which it may be, I don't know, then you probably need even more sodium than the average person because drinking extra water flushes out more sodium.