So, I installed /e/ via Easy Installer on Galaxy S9+ but now it's on Android 8.1?

No !
as this page is written for orthers devices that use fastboot commands.

I am not sure you have to reinstall stock-samsung-android-10 as you already were on this version previously. (easier using windows Odin than Linux Heimdall)

Then compare the steps of this page
https://doc.e.foundation/devices/star2lte/install
(Just don’t care about the vendor patch that was for android-8)
To the detailed steps of this illustrated guide
/[HOWTO] install /e/ on a Samsung device from a GNU/Linux PC successfully

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