1) install rom aosp_Q_F5122_11092019-eng.zip with TWRP
For gapps to work you have to flash also Magisk, maybe that's what creating the bootloop. Also this version of AOSP is really old and very experimental, maybe you should try a newer one (don't know if it could influence the bootloop or not). Also always format use data (NOT WIPE) when installing a different version of Android (for example from 9 to 10 or vice versa).
Because as stated in the first post this ROM was only a test for the new 10 features and nothing more. If you want to try aosp 10 you can find it here
Disclaimer: this is not self promotion or denigration of the OP work. It is a far better developer than me and many others.I ended up with the splash screen Sony and that's it. I followed this https://www.getdroidtips.com/android-10-q-sony-xperia-x article
which is based on this guide. My phone is F5121.
Thank you for getting back to me. Could you please drop a link here on guide I can follow to get Android either 10 or 9 working on my F5121 ? At the moment the phone is just useless.Disclaimer: this is not self promotion or denigration of the OP work. It is a far better developer than me and many others.
That being said: this build is really outdated. You should use the one provided by me or even better the one by vknecht that are, at least updated to last android 10 security patch released (still old but not so much). This, as stated by OP was just an early experiment and nothing more
fastboot flash oem SW_BINARIES_FOR_XPERIA_ANDROID_9.0_2.3.2_V9_LOIRE.img
There is no kernel-dtb-suzu in kernel/sony/msm-4.14/common-kernel, that's why. They haven't provided one yet.
You could force it to build by changing BUILD_KERNEL := false to BUILD_KERNEL :=true in /././kernel/sony/msm-4.14/common-kernel/KernelConfig.mk
It would help if "they" provided us with a more stable kernel to build in the meantime. The repo only has 4.14. Can with just clone the pie 4.9 kernel and build with that?