So, had a little free time the day before yesterday...
1. Gathered all needed pieces and software. Using pure 64bit Mageia Linux here, so made sure, that I had 64bit make_ext4fs.
2. Started to unlock bootloader. Remembered that Mageia's fastboot is named "fastboot-android" First try was unsuccessfull - realised that "usb debugging" is not enough, also "unlock bootloader" should be selected in android setup. Now unlocking succeeded. Did not care about DRM keys etc
3. Without disconnecting the Xperia X after unlocking bootloader, changed to the directory where I had all sailfishosx files. Made symlink "fastboot" pointing to "fastboot-android" and as sudo is not used in Mageia got root privileges with su. Started the flash.sh script. Went to get some coffee as the flashing should take several minutes, but when I returned, the process was over.
4. Started Xperia X with SailfishOS. Initial configuration & tutorial. Seems to be OK. Turned the phone off.
5. Yesterday started following tips&tricks updateing the software, enabling bluetooth, doubletap etc.
6. Upgraded to 2.1.2.3
7. Installed estonian-community-translation-jolla - thanks to the Estonian translation team
8. Took out SIM card from Nexus6&LineageOS14.1 and inserted in Xperia X and lived with it as my primary phone until today morning.

Some additional remarks:
1. 128GB SD card (exfat) was not mounted even with the "Bleeding edge sd-utils". Changed the filesystem to vfat and that was OK.
2. Only 3 Ambiences are selectable.
3. Doubletap wake does not work all time at the first try. Usually there are several attempts needed.
4. Opening e-mail's for reading is very slow...
5. During the call the volume is very low even at maximum level. Not usable at street noise level...
5. No internet connection sharing (actually a showstopper here)
6. xmpp conversation options are very basic - no support for "message carbons" or "message archive management". Is it possible to join xmpp conferences?
7. Not possible to install any openrepos.net client (warehouse, storeman)

Finally - of course many thanks to m4r0v3r for creating the image and also BluesLee and others for valuable information to workaround issues