Not sure if you're still shopping, just wanted to chime in and say the S8 is still a great phone. I'm trying to talk my mom out of ditching hers. It still performs great, camera is nice, solid build, okay battery. The main downside is not getting the latest android, though I think some of them will run Pie. For a lot of people, that probably doesn't matter much unless there's a specific feature of the newer android that you absolutely must have. Otherwise, I'd say go for that older flagship. It'll still beat out your current midrange for at least some of the important stuff and keep up with the rest.
That said... quick charge is nice, and while it is a little harder on lithium batteries...its less than you'd think. I've been vaping for years... for those who don't know much about vaping, the batteries used often feature the same chemistry, with only the structure of the cells differing (single cells in series/parallel groups versus piles for phones.)
For many of them I was pushing the continuous current limits for those batteries. If they're rated 30A, I'll do 35-40
When you're doing this, the voltage dips out faaaast. You really have to be careful because if you discharge them too quickly, or down to too low of a voltage, they can actually melt down violently. This has greatly improved over the past decade, but the mechanics are fundamentally the same. Current pull hurts them more than anything. Each and every discharge brings them closer to death. This goes for all lithium chemistry cells. It's simply a matter of degrees. Batteries I designate for over-current don't last more than 6 months on average. Those same batteries in my more normal setups pulling maybe 10-15A will hold up for 2 years.
Which brings me to my first point, which is that the biggest cooker of batteries in this family is discharge. The more power-intensive your usage, the sooner the batteries start losing capacity. The harder they're pushed down, the faster they degrade. This means your high-end phones will always slip back sooner. Something to consider if battery life matters. If you're going to buy a good-performing phone, expect mediocre battery life. The technology isn't there for more than you're getting. More likely we'll see more efficient high-performance chips before the batteries themselves improve significantly.
Charging is the other side of things. Most good multi-chargers allow you to designate the charging current, usually .5A to 2A for lithium. In fact, your phone is typically the same. Slow/standard charge would be ~.5A-1A while turbo/rapid/jizzum-cannon/whatever charge is usually ~2A, maybe a smidge more. Why do these external chargers feature these low-current options? Well... a lot of people say its for longevity, but the truth is, that's not so true anymore... or more it is not significant in practice. There is degradation, but it's not what's going to kill the cell first. Modern lithium can take high charging current much better that past iterations. It still hurts them, but it's kind of a drop in the pan compared to other factors, such as a lack of ventilation/cooling under high discharge.
But it is still a.... lets say "lower quality" charge. A battery charged at .5A will hold voltage under load significantly longer than one charged at 2A. I can tell you for my vapes, I've never charged at 2A unless in a pinch, because it dies very quickly when I do. If you fast charge all of the time, you'll typically find that your battery doesn't hold up how you might like. Your phone becomes addicted to it, in a sense. But that doesn't mean there's anything wrong with the battery. Try a cheap cable overnight and watch how much longer it lasts the next day. I used to always charge my vape batteries at 2A. I didn't stop because the batteries were becoming unsuitable sooner... I wasn't ripping through them faster. The time between charges was just significantly shorter.
One other minor thing... battery ratings are absolutely worthless. Generally, the parameters for testing are determined by manufacturer, just like the advertised runtime. Meaning if they want to boast a number, they can test for that number. Even big-name manufacturers have been caught on this. If you really want to know how your phones battery will do, you need to pursue 3rd party testing.
Honestly, fast charging is a godsend in modern phones. If you have a high-performance phone, I'd say it's a vital feature. Just know that it's going to be harder on your batteries to some degree and that a fast charge will also deplete faster than a slow one and slow charge whenever possible for optimal battery life. I would never tell anyone not to use it. If you have it, use it. But understand that it is a compromise on battery life, if not over the entire service life, then at least that charge cycle. So it's not the best, but it is the best that we presently have.
Megapixels is another one of those stupid bullshit things. If you're simply sharing images to be viewed on a phone or on a monitor, 12 is really plenty. Even 8-10 is fine. An 8mp image is still HUGE for a screen and will be scaled down to 50% at a minimum, for FULL SCREEN. IF you're pixel-peeping, you'll see obvious differences, but scaled-down, those differences tend to disappear. In fact, a higher MP camera with the same size sensor as a lower MP camera will sometimes have MORE noise and detail loss due to the higher pixel density. The pixels on the lower MP sensor are larger, meaning each pixel can gather more light. Since they're more sensitive, they don't have to be pushed as hard in order to get the exposure. This is important when you're dealing with a sensor that is already tiny, and is why newer phones with lower MP counts still take better-looking pictures (alongside better optics and more sophisticated post-processing.)
Megapixels matter most when it comes to print, where you usually want the highest DPI possible for larger prints. The only worthwhile consideration with MP on a phone camera is cropping/digital zoom. With more resolution, you can zoom more. Professional photographers and enthusiasts like to have more because it aids in editing, giving you more flexibility in many ways. But you're losing half of those by the time your phone finishes processing the image anyway.
Basically, it's a bunch of oversimplification. What matters with cameras is the quality of the lens, sensor size/quality, sensitivity, processing... so many other unspecified factors mean everything. And this stuff is where the real money is. It doesn't cost more to make a lower-quality, higher MP sensor and put it behind an inferior lens, though it sure looks better on a specs list. Always look at sample images. That'll tell you more about the camera than any of the numbers or features tossed at you.