Na tessék, megtaláltam XDA-n: mindenképpen a boot partícióra kell majd a TWRP-t flashelni ELŐSZÖR. Ezt írta AdrianDC (itt: https://forum.xda-developers.com/t/recovery-official-ubl-twrp-3-5-0-touch-recovery-for-xperia-sp.3309938/page-13):

"As you found out, Sony 8960 devices, in this case Xperia SP Huashan, do not have a standard recovery bootloader.

We install the recovery in the FOTA partition and use a specific init sequence to boot the recovery out of FOTA.

Upon first installation, TWRP (or Lineage Recovery for that matter) has to be installed in the Boot partition through fastboot in order to have a first step into flashing the FOTA, as you did, and to format / install the new ROM.

Having overwritten the boot partition, you can't boot the Stock ROM.
You'd either require the original ROM's bootimage, or start using a new ROM.

We detailed everything extensively in the LineageOS Wiki for each Sony devices : https://wiki.lineageos.org/devices/huashan/install"

Ezt a wiki-s linket ajánlom is elolvasásra, mert ebben is van egy módszer leírva a TWRP feltolására. Érdemes akkor már rögtön a ROM+GAPPS-t is telepíteni, azaz az SD kártyán legyen ott a ROM és a GAPPS is előre (132MB): https://mega.nz/folder/e9NhVARL#N-csHttthHy0wUJjMI_Ekw