(I do half remember this sort of situation with FP3, but not the detail of exactly what made the upgrade so problematic that someone gave that advice; perhaps it was always felt that an easier route would “become available soon”.)
Maybe reading this tutorial [HOW-TO] Flash /e/-OS on Fairphone 3 using Debian based GNU/Linux would give you confidence to try a manual install. The ROM which you will download now contains a script which makes the ROM almost self installing, so debugging Easy Installer maybe harder than running the script.
The ROM contains its own version of platform-tools and is suitable for a Windows or Linux install.
Regarding Easy Installer and adb, Easy Installer contains its own route to platform-tools via snap so your system install of platform-tools is not required.
This link Run apps on a hardware device | Android Studio | Android Developers shows the details to make platform-tools work nicely in Debian, under the section heading:
2. Set up your system to detect your device.
- Ubuntu Linux: Set up the following: