[Xiaomi Mi A2 / 6X Questions & Answers] User699:
Thank you very much for answering @tecknight !

1. Yes, all that works perfectly.

2. Currently I have the official lineage recovery installed which was the newest when I flashed my ROM for the very first time. It's called lineage-17.1-20201114-recovery-jasmine_sprout.img.

I booted TWRP with fastboot, that's true. I am using an unofficial twrp which I got somewhere from here. It says 3.4.0-0 is its version (official twrp didn't boot).

3. Here's the output from ls /dev/block/by-name -l :

total 0
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 abl_a -> /dev/block/mmcblk0p25
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 abl_b -> /dev/block/mmcblk0p26
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 apdp -> /dev/block/mmcblk0p11
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 bk1 -> /dev/block/mmcblk0p51
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 bluetooth_a -> /dev/block/mmcblk0p38
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 bluetooth_b -> /dev/block/mmcblk0p50
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 boot_a -> /dev/block/mmcblk0p62
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 boot_b -> /dev/block/mmcblk0p63
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 cache -> /dev/block/mmcblk0p64
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 cmnlib64_a -> /dev/block/mmcblk0p22
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 cmnlib64_b -> /dev/block/mmcblk0p24
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 cmnlib_a -> /dev/block/mmcblk0p21
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 cmnlib_b -> /dev/block/mmcblk0p23
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 ddr -> /dev/block/mmcblk0p28
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 devcfg_a -> /dev/block/mmcblk0p30
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 devcfg_b -> /dev/block/mmcblk0p31
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 devinfo -> /dev/block/mmcblk0p43
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 dip -> /dev/block/mmcblk0p27
lrwxrwxrwx 1 root root 20 1972-03-24 14:06 dpo -> /dev/block/mmcblk0p2
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 dsp_a -> /dev/block/mmcblk0p48
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 dsp_b -> /dev/block/mmcblk0p49
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 frp -> /dev/block/mmcblk0p14
lrwxrwxrwx 1 root root 20 1972-03-24 14:06 fsc -> /dev/block/mmcblk0p3
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 fsg -> /dev/block/mmcblk0p45
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 hyp_a -> /dev/block/mmcblk0p17
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 hyp_b -> /dev/block/mmcblk0p18
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 keymaster_a -> /dev/block/mmcblk0p36
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 keymaster_b -> /dev/block/mmcblk0p37
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 keystore -> /dev/block/mmcblk0p13
lrwxrwxrwx 1 root root 20 1972-03-24 14:06 limits -> /dev/block/mmcblk0p4
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 logdump -> /dev/block/mmcblk0p57
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 logfs -> /dev/block/mmcblk0p44
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 mdtp_a -> /dev/block/mmcblk0p52
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 mdtp_b -> /dev/block/mmcblk0p53
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 mdtpsecapp_a -> /dev/block/mmcblk0p34
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 mdtpsecapp_b -> /dev/block/mmcblk0p35
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 minidump -> /dev/block/mmcblk0p61
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 misc -> /dev/block/mmcblk0p42
lrwxrwxrwx 1 root root 18 1972-03-24 14:06 mmcblk0 -> /dev/block/mmcblk0
lrwxrwxrwx 1 root root 22 1972-03-24 14:06 mmcblk0rpmb -> /dev/block/mmcblk0rpmb
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 modem_a -> /dev/block/mmcblk0p58
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 modem_b -> /dev/block/mmcblk0p59
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 modemst1 -> /dev/block/mmcblk0p46
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 modemst2 -> /dev/block/mmcblk0p47
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 msadp -> /dev/block/mmcblk0p12
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 oops -> /dev/block/mmcblk0p10
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 persist -> /dev/block/mmcblk0p55
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 persistbak -> /dev/block/mmcblk0p56
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 pmic_a -> /dev/block/mmcblk0p19
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 pmic_b -> /dev/block/mmcblk0p20
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 rawdump -> /dev/block/mmcblk0p60
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 rpm_a -> /dev/block/mmcblk0p15
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 rpm_b -> /dev/block/mmcblk0p16
lrwxrwxrwx 1 root root 20 1972-03-24 14:06 sec -> /dev/block/mmcblk0p6
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 splash -> /dev/block/mmcblk0p54
lrwxrwxrwx 1 root root 20 1972-03-24 14:06 ssd -> /dev/block/mmcblk0p5
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 sti -> /dev/block/mmcblk0p39
lrwxrwxrwx 1 root root 20 1972-03-24 14:06 storsec -> /dev/block/mmcblk0p9
lrwxrwxrwx 1 root root 20 1972-03-24 14:06 switch -> /dev/block/mmcblk0p1
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 system_a -> /dev/block/mmcblk0p65
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 system_b -> /dev/block/mmcblk0p66
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 toolsfv -> /dev/block/mmcblk0p29
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 tz_a -> /dev/block/mmcblk0p32
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 tz_b -> /dev/block/mmcblk0p33
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 userdata -> /dev/block/mmcblk0p69
lrwxrwxrwx 1 root root 20 1972-03-24 14:06 vbmeta_a -> /dev/block/mmcblk0p7
lrwxrwxrwx 1 root root 20 1972-03-24 14:06 vbmeta_b -> /dev/block/mmcblk0p8
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 vendor_a -> /dev/block/mmcblk0p67
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 vendor_b -> /dev/block/mmcblk0p68
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 xbl_a -> /dev/block/mmcblk0p40
lrwxrwxrwx 1 root root 21 1972-03-24 14:06 xbl_b -> /dev/block/mmcblk0p41

4. Yes, I can boot into fastboot and run fastboot commands.

5. Done.

Edit: I just booted your linked version of twrp. It asked me for a password first (all versions I tried didn't) and after I typed in my password it was able to decrypt my userdata! I'll wait for your instructions, but I'll just create a backup with every single entry that I can select there. Thanks already!