[update]
Hi guys. So, I managed to fix it!
I gave it another shot and it took me a whole evening of fiddling around with the settings of both the Pi and Kodi, but eventually I found the solution that made it work with the latest nightly build of LibreElec 10.
It basically revolves around the config.txt which I adjusted so that the Pi doesn't call the EDID to set it's video settings (if I understand correctly). After I got 2160p/24 to work via the reciever's passthrough, some video's didn't play. Just a black screen with only sound. That was solved by upping the GPU Memory to 256MB with the gpu_mem=256 line in the config.txt file (setting it to gpu_mem=512 prevents my Pi4 to boot for some reason, although it's a maximum setting that is supported).
I also set the whitelist in Kodi to 2160p/24 as the only resolution. Now, every video plays in full 2160p! Unfortunately my receiver doesn't support HDR passthrough, so I'll probably replace my STR-DN1050 with the STR-DN1080 which does support 4K HDR passthrough. But for now, I'm a happy camper
Here's my working config.txt:
# SPDX-License-Identifier: GPL-2.0-or-later
# Copyright (C) 2009-2014 Stephan Raue ([email protected])
# Copyright (C) 2016-present Team LibreELEC (https://libreelec.tv)
################################################################################
# Bootloader configuration
# config.txt version v1 (do not remove or change this line!)
################################################################################
# For more options and information see
# http://rpf.io/configtxt
################################################################################
# Default GPU memory split, 76MB are needed for H264 decoder
gpu_mem=256
# Disable Firmware parsing of EDID
disable_fw_kms_setup=1
# HDMI tweaks
hdmi_ignore_edid=0xa5000080
hdmi_force_hotplug=1
hdmi_force_edid_audio=1
################################################################################
# Include distribution specific config file if it exists.
################################################################################
[all]
include distroconfig.txt
# uncomment to enable infrared remote recevier connected to GPIO 18
#dtoverlay=gpio-ir,gpio_pin=18
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I hope I can help someone else who ran or runs into the same problem. This thread can now be marked as SOLVED.