Failing to flash custom (Magisk-patched) boot.img to Samsung S20 FE (4G) with Heimdall

I am attempting to flash a Magisk -patched boot.img from Linux onto my unlocked SM-G780G. I have so far tried doing that using fastboot flash ... to which the device responded that it doesn't know the command. The device is a Samsung Galaxy S20 FE (SM-G780G). I then tried Heimdall v1.4.2 (on Ubuntu 20.4) and got this:

$ sudo heimdall download-pit --output bla.pit --verbose --stdout-errors
Heimdall v1.4.2

Copyright (c) 2010-2017 Benjamin Dobell, Glass Echidna
http://www.glassechidna.com.au/ 

This software is provided free of charge. Copying and redistribution is
encouraged.

If you appreciate this software and you would like to support future
development please consider donating:
http://www.glassechidna.com.au/donate/ 

Initialising connection...
Detecting device...
      Manufacturer: "Samsung"
           Product: "SM8250"

            length: 18
      device class: 2
               S/N: 0
           VID:PID: 04E8:685D
         bcdDevice: 0100
   iMan:iProd:iSer: 1:2:0
          nb confs: 1

interface.altsetting: num endpoints = 1
   Class.SubClass.Protocol: 02.02.01
       endpoint.address: 82
           max packet size: 0010
          polling interval: 09

interface[1] .altsetting: num endpoints = 2
   Class.SubClass.Protocol: 0A.00.00
       endpoint.address: 01
           max packet size: 0400
          polling interval: 00
       endpoint[1] .address: 81
           max packet size: 0400
          polling interval: 00
Claiming interface...
Setting up interface...

Initialising protocol...
ERROR: libusb error -7 whilst sending bulk transfer.ERROR: libusb error -7 whilst sending bulk transfer. Retrying...
 Retrying...
ERROR: libusb error -7 whilst sending bulk transfer.ERROR: libusb error -7 whilst sending bulk transfer. Retrying...
 Retrying...
ERROR: libusb error -7 whilst sending bulk transfer.ERROR: libusb error -7 whilst sending bulk transfer. Retrying...
 Retrying...
ERROR: libusb error -7 whilst sending bulk transfer.ERROR: libusb error -7 whilst sending bulk transfer. Retrying...
 Retrying...
ERROR: libusb error -7 whilst sending bulk transfer.ERROR: libusb error -7 whilst sending bulk transfer. Retrying...
 Retrying...
ERROR: libusb error -7 whilst sending bulk transfer.ERROR: libusb error -7 whilst sending bulk transfer.

ERROR: Failed to send handshake!ERROR: Failed to send handshake!ERROR: Failed to receive handshake response. Result: -7
ERROR: Failed to receive handshake response. Result: -7
ERROR: Protocol initialisation failed!

ERROR: Protocol initialisation failed!

Releasing device interface...

I've tried both with and without superuser context (sudo) and the result is always the same. I am also unable to retrieve the PIT file from the phone. However, the boot loader is successfully unlocked as I can see in the freshly initialized (due to the data wipe) Android, while running, and also during boot (shows a warning regarding the unlocked state, which has to be dismissed before the boot continues). I am successful in entering ODIN/download mode (holding both Volume keys _while_ the phone restarts, followed by Volume+), but still nothing seems to work when attempting to communicate with the phone. I also tried using JOdin3, which detects the device and all, but already fails to retrieve the PIT file as well. I suppose it must be using Heimdall under the hood (?) ... What part am I missing? Has the procedure somehow changed for newer Samsung phone models? ---------- NB: in order to get fastboot mode to work (which I had tried before), I had to disconnect a Yubikey that I was using on the system. After that fastboot devices properly showed the phone, but using fastboot flash ... was equally unsuccessful (I tried fastboot flash boot and fastboot flash boot_a to no avail). Prior to unplugging it (although connected to a different root hub), it failed to detect the phone in fastboot mode.