What you need to take into account is:
1.1 Temperature
1.2 State of charge
2. Charge speed
All of them are really important, but some factors like charge speed and temperature are easier to control.
- You want to use fast charging tech like QC 3.0 and USB PD 2.0+ as few times as possible, like with your G6s 15W fast charging. Not only does fast-charging hurt cycle life just because it charges the cell faster, but it can also create noticeable heating due to cell internal resistance and the buck converter inside of your cellphone.
Basically, you want to charge from a regular USB power supply without fast charging, like from a 2A USB power supply, which will usually give out 7,5W-10W max depending on your phone, which will help out with cycle life quite a bit, since you won’t be pushing your phone’s battery quite as much, and less heat is produced. Or just charge from a computer’s USB port.
You don’t need to always quick charge your phone, do you? Haha 
- When charging, keep on your phone on a solid surface. The surface will act like a heatsink, and spread out the heat generated during fast charging, which can help during the morning for example if you need a few more %.
- Here’s the more complicated part now, in relation to keeping your phone between 20–80/90 as seamlessly as possible:
If you have an unlocked phone, you can try unlocking the bootloader, and getting root access to your phone. This will allow you to install a root app called Battery Charge Limit:
This will allow you to limit the max charging voltage on your phone. This can help immensely in terms of cycle life, and even fast charging more often.
I personally keep it at 86% for optimal balance between capacity/cycle life, which has helped out a ton of people in my entourage keep their phone’s battery healthy, even with quick charging.
- Final tip: If you can activate battery saver at a certain percentage, put it at 20%. This will help.
TLDR:
1. Fast charge your cell phone as few times as possible. Use a regular 2A charger, or a computer’s USB port.
2. Keep your phone on a surface while fast charging. This will help it dissipate the heat produced.
3. Limit your max charging to 80-90% charge. This will help. If you have access to root, with an unlocked phone, you can install this app to automate it, and make it as seeamless as possible:
4. Keep your phone above 20% preferably, and above 15% if you really need it.
