Hi @Kohryl,
I installed /e/OS on several Pixel 5 but always without the easy installer and therefore still via the command lines. With the right usb-c cable (a short cable) everything went very well in just a few minutes 
It’s very easy.
I followed those instructions: https://doc.e.foundation/devices/redfin/install .
I didn’t have to worry about which version of Android was already on the phone.
I would add a detail mentioned in this post Pixel 5 stuck in Fastboot Mode - #16 by PierreT and read in other messages on this forum. The installation is done on a slot, “a” or “b” (refer to the “Install /e/OS” chapter). After step 7, the recovery ask me to reboot to perform another install. I choose yes and then get back in recovery with the other active slot. I launch the install command again:
adb sideload downloaded_file_name.zip
I don’t know if it’s mandatory but it works well for me.