Alternatives for android updates for Sailfish X?

Thanks for your suggestions. Emma tool does not work: when I launch it, it recognizes my xperia but then it disables all GUI components and says I should upgrade my device under www.sonymobile.com/update (which of course does not work, since it says I am already up-to-date). I will have a try with XDA Xperifirm.

then you really should give flashtool & xperifirm a try. there's no need to hold on your customization, means you can flash another firmware(for the x of course) with higher version number.

I managed to succeed and report here for other users that might experience similar issues. I think the main issue resides in the documentation of sailfish X and the not so clear error messages on the Emma Tool. Sailfish X Instructions: At the beginning they say: "Note: If you are a Linux or macOS user, your device’s Android version is lower than 34.3.A.0.228 and you have already unlocked your device’s bootloader, then you will need to use Windows for one step of the flashing procedure." In my case: bootloader was not unlocked and lower than 34.3.A.0.228. And there is no explicit information for that case. Emma Tool: When I plugged in my phone it said I need to update. Then, below, it's written my phone is locked. I did not see it the first time.

So, what I needed to do was to unlock the bootloader first. Unfortunately I bricked my phone before by using Flashtool and xperifirm (probably because I did not unlock the phone first?) but with Sony Xperia Companion I could repair it and reinstall the stock Android. Summarizing this are the steps if you are in the same situation:

1. Unlock your bootloader
2. Use Emma Tool to upgrade to the requested Android version

Thanks for the help guys!