There is always some risk when flashing and upgrad, but it's generally quite safe.
You can do it wirelesly, but keep in mind the following:
You must allow the device to reset to defaults (uncheck the "keep settings" checkbox) since the config in the GL-inet firmware is not compatible with official OpenWrt.
Wifi is disabled by default in OpenWrt. You will need an Ethernet connection to configure OpenWrt such that you can setup and turn on the wifi.
doubt I would want to go back to stock GL.iNet OpenWrt, but if I ever wanted to return to the GL.iNet OpenWrt fork, is it as simple as uploading the stock GL.iNet sysupgrade file in vanilla OpenWrt, similar to how I download OpenWrt vanilla on GL.iNet's fork?
my main reasons for installing vanilla openwrt is becuse i dont trust propriatary software and the gl inet openwrt fork has meny propriatary addons
According to that note, you will need factory.bin when upgrading from the stock firmware. The sysupgrade.bin would force you to go through the u-boot debricking again, unless something has been changed in the stock firmware meanwhile. After the initial upgrade, sysupgrade.bin will work; I never upgraded between OpenWrt builds using anything else.