I will release here my updated "boot.img" files for all Xperia XA Ultra.
You must have an unlocked bootloader to flash with Fastboot.
You don't need to be on the same firmware as the boot image that you want to flash.
You have to flash SuperSu again from TWRP just after flashed new boot to avoid bootloop. Not needed on 33.2.B.4.70 and upper.
All boot images have "ro.secure=0" and "ro.debuggable=1" in default.prop file
and "forcencrypt" of internal memory switched to "encryptable" in fstab.mt6755 file, "dm-verity" is disabled by default.
They will probably not work on upper firmwares than the one they are built.
The Fastboot command is:
fastboot flash boot NAMEOFIMAGE.img
2017/07/26: From 36.1.A.0.182 kernel sources and Ramdisk from 36.1.A.0.182 FR firmware.