Use xiaomi adb/fastboot tools to remove apps that you don't need. This list has the apps that are safe to uninstall. Someone in the comments has listed what the functions of these apps are and better replacements (if needed)
16gb is enough for gaming for now. You can get 2 dimms of 8gb now and upgrade to 4 dimms of 8gb later. This will stop the performance bottlenecks from happening.The motherboard you're going to buy will have 4 dimm slots so there's no issues :)