One thing that I'm concerned with, is longevity when a person is on keto. Do you share this concern? Do you know any reliable data source on keto diet's impact on longevity?
I have no concern because there's nothing about a low carb diet that restricts any essential nutrients. On evolutionary time scales, consuming a significant amount of carbs is a recent and unprecedented change to human diets. There are three types of macro-nutrients: protein, fat and carbs. For most of history our hunter/gatherer ancestors ate far more protein and fat than carbs, which is why they were in a ketogenic state most of the time. It's only in the last hundred years in Western democracies that carbs have become overwhelmingly dominant in human diets. In terms of long-term dietary impact, highly-processed manufactured foods and intensive factory farming (which are mostly carbs) are the massive uncontrolled experiment on the broad population that's worth being concerned about. That's the new thing for which there's no long-term data whereas keto has tens of thousands of years of proven success in humans. Going from the Standard American Diet to keto is opting out of the uncontrolled experiment and returning to what we know works. Chronic obesity and Type II diabetes are diseases of diet and they weren't common in humans until the last hundred years.
As for my personal health and longevity, keto has certainly added at least a decade to my life because before keto I was significantly overweight and had been diagnosed with pre-diabetes, high blood pressure, high triglycerides and bad HDL/LDL. I also had chronic sleep apnea as well as IBS/GERDS. I was on five prescribed medications to treat these issues. By the end of the first year on keto those serious health problems had completely resolved and I was off all five medications. To be clear, keto itself isn't some miracle cure. These problems were caused by my obesity and unhealthy diet. Keto helped me not be obese and to eat a healthier diet richer in nutrients and with far fewer manufactured and processed foods. My previous diet was making me sick.
May I ask what the differences are between the keto diet you used and the low carbohydrate diet that you are on now? Do you allow for more carbohydrates than a typical keto diet?
I suggest you read the FAQ of the Reddit keto forum: https://www.reddit.com/r/keto/wiki/faq. I recently posted my personal "Keys to Keto Success" learned during my 8 month weight loss 8 years ago: https://www.hackerneue.com/item?id=44685689 which includes some effective hacks I discovered. The biggest thing is to do keto rigorously. It's different than most any other diet because it's not just about reducing calories. The power of keto is 100% in cutting carbs, which is why I advise "Cheat all you want on calories but never (ever) on carbs".
For me, cutting calories on any diet was always difficult, painful and exhausting. Keto worked because I only focused on cutting carbs. Once I got my blood sugar under control by limiting carbs, my raging hunger just evaporated and the calories pretty much took care of themselves. The change was dramatic. After the first month I was no longer tracking calories to stop over-eating but instead to ensure I was getting enough :-). During the second month there was a point where I was slightly concerned I might be losing weight too fast because I was never hungry, which was certainly never something I'd imagined would be a problem for me. Of course, that's easily addressed by increasing calories a bit.
There's just one thing to watch out for as you do keto. On keto you're able to eat a lot of fats which are very filling (and delicious). This is wonderful but it's only possible because having almost no carbs puts you in a ketogenic state where instead of burning glucose (carbs) for energy as you do now, your body burns those fats for energy instead of storing them (weight gain). So, unlike a normal calorie-reduction diet, where you can slip one day and just get back on the wagon the next day, cheating on carbs during keto will convert all that delicious fat you're eating into not only pounds of weight but potentially into all the bad stored fat related things like vascular plaque etc. So, basically, keto is the one diet where cheating doesn't just "delay your weight loss another day", it changes from magically protecting you from the negative effects of eating huge amounts of fat to actively making it much worse. Eating all that luscious fat thinking you're protected by Keto but then regularly cheating on carbs "just a little" (as many do on other diets), is pretty much the worst diet imaginable. You'll turbo-charge weight gain AND all the negative effects of excess fat consumption (cholesterol, lipids, A1C, etc). Theoretically, aggressively eating keto-levels of fat but NOT being in a ketogenic state to burn it off could, given time, make you pre-diabetic and your lipid blood panels will freak your doctor out.
So, IMHO, keto is wonderful but there's just one unbreakable rule. You really, really can't cheat on keto (even a little). When I first understood this I was a little concerned but as I researched more it made complete sense. Keto isn't magic. With all these benefits of easy weight loss and healthy metabolism of course there's a potential downside to avoid. Now you know what it is - so go in with eyes wide open. For me, the stringent, "can't cheat, ever" (on carbs) aspect of keto was actually a psychological benefit. It made it 100% binary and thus easy to fully commit. I'm either really doing this or I shouldn't bother. Putting tangible health consequences on regularly "slipping just a little" actually helped me stick with keto rigorously during the first few weeks of transition and habit change (which is really the only hard part, after 30 days keto gets much easier). But you may be different, so you just need to decide if you can really commit 100% for the first ten days. If you manage that, then set your goal to finish the month. The rest really is all downhill from there. If you're as overweight as I was, after the first month of strict keto your weight loss will be so obvious people will definitely notice. In the second month, my admin told me people at the office were asking her if I was "okay", like they were worried I had cancer or was on chemo :-). So I started sharing a little more widely that I'd changed my eating habits. At least nowadays most people have heard of keto.