What precautions to take when flashing Marlin 2.0?
It wasn't advised to use the version 2.x because it was in development for 32-bit micro processors. Now that it has been released as the official version, you can use it for 8-bit micro processors.

But, it totally depends on the amount of options in Marlin you activate (bed leveling, advanced menu, M5xx, etc.). Luckily you can see how large the installation is after you have built it e.g. in PlatformIO. Also, in the configuration files frequently is mentioned how much extra storage activating an option costs (search for PROGMEM in the Marlin sources). Unless you want all options active, you'll be fine. I'm running it on an AVR (MEGA2560) for a CoreXY with bed leveling and some more options; it runs fine.