[General Questions and Answers] M47Z:
Thanks for the reply! Yes, I am sure it's f2fs. The only reason I had to add the "iirc" is because it has been running so smoothly for so long, I could not be 100% sure.

But all messages etc fit the picture, feg dmesg through adb in recovery:

[ 9130.186284] F2FS-fs (sda18): Magic Mismatch, valid(0xf2f52010) - read(0xfed95610)
[ 9130.186288] F2FS-fs (sda18): Can't find valid F2FS filesystem in 1th superblock
[ 9130.186424] F2FS-fs (sda18): Magic Mismatch, valid(0xf2f52010) - read(0xe1183ecd)
[ 9130.186430] F2FS-fs (sda18): Can't find valid F2FS filesystem in 2th superblock

... after failed boot and whenever I try to mount data. I get exactly the same message when I try to mount the copied partition on linux. Including the mismatching magic number. At least that makes me hopeful I copied the right partition. When I try to mount the partition image as ext4 I get:

[131036.109835] EXT4-fs (loop14): VFS: Can't find ext4 filesystem

Does not look like ext4. Also, /etc/fstab looks the same:

/dev/block/dm-0 /system_root ext4 ro 0 0
/dev/block/dm-3 /system_ext ext4 ro 0 0
/dev/block/dm-2 /product ext4 ro 0 0
/dev/block/dm-1 /vendor ext4 ro 0 0
/dev/block/sda13 /cache ext4 rw 0 0
/dev/block/sda7 /cust ext4 rw 0 0
/dev/block/sde4 /firmware vfat ro 0 0
/dev/block/sda2 /persist ext4 rw 0 0
/dev/block/mmcblk0p1 /sdcard1 exfat rw 0 0
/storage ext4 rw 0 0
/usb_otg auto rw 0 0
/dev/block/sda12 /metadata ext4 rw 0 0
/dev/block/sda18 /data f2fs rw 0 0

And this is /etc/additional.fstab:

# Copyright (c) 2018-2019, The Linux Foundation. All rights reserved.
#
# Redistribution and use in source and binary forms, with or without
# modification, are permitted provided that the following conditions are
# met:
# * Redistributions of source code must retain the above copyright
# notice, this list of conditions and the following disclaimer.
# * Redistributions in binary form must reproduce the above
# copyright notice, this list of conditions and the following
# disclaimer in the documentation and/or other materials provided
# with the distribution.
# * Neither the name of The Linux Foundation nor the names of its
# contributors may be used to endorse or promote products derived
# from this software without specific prior written permission.
#
# THIS SOFTWARE IS PROVIDED "AS IS" AND ANY EXPRESS OR IMPLIED
# WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NON-INFRINGEMENT
# ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS
# BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
# CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
# SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
# BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
# WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE
# OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN
# IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

# Android fstab file.
# The filesystem that contains the filesystem checker binary (typically /system) cannot
# specify MF_CHECK, and must come before any filesystems that do specify MF_CHECK

#<src> <mnt_point> <type> <mnt_flags and options> <fs_mgr_flags>
system /system ext4 ro,barrier=1,discard wait,logical,first_stage_mount,avb=vbmeta_system,avb_keys=/avb/q-gsi.avbpubkey:/avb/r-gsi.avbpubkey:/avb/s-gsi.avbpubkey
system_ext /system_ext ext4 ro,barrier=1,discard wait,logical,first_stage_mount,avb
product /product ext4 ro,barrier=1,discard wait,logical,first_stage_mount,avb
vendor /vendor ext4 ro,barrier=1,discard wait,logical,first_stage_mount,avb=vbmeta
/dev/block/bootdevice/by-name/cache /cache ext4 nosuid,noatime,nodev,barrier=1 wait
/dev/block/bootdevice/by-name/boot /boot emmc defaults defaults
/dev/block/bootdevice/by-name/recovery /recovery emmc defaults defaults
/dev/block/by-name/metadata /metadata ext4 noatime,nosuid,nodev,discard wait,check,formattable,first_stage_mount
/dev/block/bootdevice/by-name/userdata /data f2fs noatime,nosuid,nodev,discard,reserve_root=32768,resgid=1065,fsync_mode=nobarrier,inlinecrypt latemount,wait,check,formattable,fileencryption=aes-256-xts:aes-256-cts:v2+inlinecrypt_optimized+wrappedkey_v0,metadata_encryption=aes-256-xts:wrappedkey_v0,keydirectory=/metadata/vold/metadata_encryption,quota,reservedsize=128M,sysfs_path=/sys/devices/platform/soc/1d84000.ufshc,checkpoint=fs
/devices/platform/soc/8804000.sdhci/mmc_host* /storage/sdcard1 vfat nosuid,nodev wait,voldmanaged=sdcard1:auto,encryptable=footer
/devices/platform/soc/1da4000.ufshc_card/host* /storage/sdcard1 vfat nosuid,nodev wait,voldmanaged=sdcard1:auto,encryptable=footer
/dev/block/bootdevice/by-name/modem /vendor/firmware_mnt vfat ro,shortname=lower,uid=1000,gid=1000,dmask=227,fmask=337,context=u:object_r:firmware_file:s0 wait
/dev/block/bootdevice/by-name/dsp /vendor/dsp ext4 ro,nosuid,nodev,barrier=1 wait
/dev/block/bootdevice/by-name/persist /mnt/vendor/persist ext4 noatime,nosuid,nodev,barrier=1 wait,check
/dev/block/bootdevice/by-name/bluetooth /vendor/bt_firmware vfat ro,shortname=lower,uid=1002,gid=3002,dmask=227,fmask=337,context=u:object_r:bt_firmware_file:s0 wait
/devices/platform/soc/a600000.ssusb/a600000.dwc3/xhci-hcd.0.auto* /storage/usbotg vfat nosuid,nodev wait,voldmanaged=usbotg:auto
# Need to have this entry in here even though the mount point itself is no longer needed.
# The update_engine code looks for this entry in order to determine the boot device address
# and fails if it does not find it.
/dev/block/bootdevice/by-name/misc /misc emmc defaults defaults
/dev/block/zram0 none swap defaults zramsize=50%

I will attatch a file that is the first MB of the userdata partition (created with "dd if=./userdata_rescue_copy.img of=./userdata_rescue_copy_trimmed.img bs=1048576 count=1").

Maybe someone can figure out what is going on! All other partitions continue to mount just fine btw.