Dear @Luksus - again thank you very much for your Effort and for having shared this new UT 20.04 Upgrade/Release!
53.001). I confirm that I was able to boot it without any workaround 
However, I needed to do a temporary Ubuntu Touch 16.04 installation in between (as shown in the [PASSED] TC002/53.301). I needed to execute the additional steps "53.309-310", to be able to boot UT20.04@X605L 4/64, after having flashed boot.img, ubuntu.img and recovery.img.
53.002). On the other hand, I was not able to boot it when I flashed these 3 images directly on top of Android 9, without the temporary UT16.04 installation (see the [FAILED] TC001/53.201 for more details). Therefore, I'm not sure if someone who has a Lenovo X605L with Android 9 would be able to install UT 20.04 without the need to install UT 16.04 first.
53.003). As you suggested, I added flashing of recovery.img and I avoided wiping (unless explicitly requested, e.g. in 53.310) and I stopped using any workaround.
53.004). In my Test Cases I wanted to reflect the real life use cases which UT users can face during Ubuntu Touch 20.04 installation. Could you let me know what/how I could change there, in order to better support you, please ?
53.005). Could I also ask you, if I correctly understood that when I have the Fresh Factory Reset Android 9 on X605L 4/64 then I should only flash boot.img, ubuntu.img and recovery.img, using the FastBootMode? or maybe I should do something more (what/how)?
53.006). I'm asking that because the UBports-Installer downloaded 9 files for UT 16.04 (I'm not sure if all of them were flashed then) but I'm asking myself if these only 3 which I flashed for UT 20.04 were not to few?
53.007). I'm happy and ready to do the re-test with more files if I could get them, together with the information what/how I should do
Dear @Luksus, thank you in advanced for your feedback and kind regards~~ Piotrek~~pe3no.
If you want to know more, see the below details please:
53.101). PREREQUISITES:
53.102). I used boot.img (MD5=e1fc16cdb3e85094ce40c96315ccc68a) and system.img (MD5=75fc62c23bd81858528c140660ab573a) extracted from artifacts.zip (ID=5951873058), shared in your Latest Build ("/50").
53.103). I took recovery.img shared in your "/52" entry (MD5=02967e01c979f04e79e10e610a22ea20).
53.104). "ETW" = "Empty-Tray-Workaround", which I was using to bypass booting problems.
53.105). "FBM"= "FastBootMode".
53.106). "TC", "TCs" = Test Case, Test Cases.
53.107). Tablet has the BootLoader Unlocked. User installs UT20.04 at X605L 4/64 (ZA490162PL or equivalent). Other models not tested.
53.108). TWRP: the file "twrp-3.7.0_9-0-X605F.img" from https://twrp.me/lenovo/lenovo_X605F.html can be installed for X605L, using FBM.
53.201). Name and Status: TC001 - [FAILED]
53.202). Description: "Android9@X605L -> via -> FBM-FlashBootSystemRecovery-NoETW -> to -> UT20.04"
53.203). Details: User of an Android 9 X605L 4/64 tablet wants to manually install Ubuntu Touch 20.04 on the device, flashing: boot.img, ubuntu.img and recovery.img in the FastBootMode. Empty-Tray-Workaround is not applied.
53.204). Test Case Steps:
53.205). I get back the Factory Fresh Android 9 on X605L 4/64.
53.206). In TWRP-3.7 I restored "20180802...FreshAndroid9" backup (which I can share with you if needed).
53.207). I booted X605L to Android 9, set up Language, accepted License Agreement and all the rest, skipping what possible to save time and to simplify the step.
53.208). Now I have the Factory Fresh Android 9 on X605L 4/64, with the Stock Recovery, because TWRP has been replaced by Android at boot because Magisk was not installed.
53.209). Therefore, now we have the real life use case - the typical, fresh, not customized Android 9 on X605L 4/64 and user wants to manually flash UT20.04. Bootloader is unlocked.
53.210). I turn off X605L, connect it to PC via USB-C cable and I switch X605L to FastBootMode (VolumeDown+Power).
53.211). I flashed boot.img, ubuntu.img, recovery.img and I restarted, being in FBM:
$ fastboot flash boot boot.img; fastboot flash system ubuntu.img; fastboot flash recovery recovery.img; fastboot reboot
53.212). Lenovo Logo appears, I disconnect USB-C cable. Ubuntu Touch is not booting. I wait 5 minutes.
53.213). I don't apply the Empty-Tray-Workaround, so both SIM and microSD are continuously inside X605L.
53.214). I turn off X605L (Power button pressed for 10 seconds).
53.215). I turn on X605L (Power button - 2 seconds).
53.216). Lenovo Logo is continuously shown on the screen and UT is not booted. I wait 5 minutes again but no success.
53.217). I consider the TC to be FAILED.
53.301). Name and Status: TC002 - [PASSED]
53.302). Description: "Android9@X605L -> via -> {temporary UBports-Installer 16.04 Installation} -> FBM-FlashBootSystemRecovery-NoETW -> to -> UT20.04"
53.303). Details: User of an Android 9 X605L 4/64 tablet temporarily installs UT 16.04 using UBports-Installer, in order to prepare the device to perform the manual installation of Ubuntu Touch 20.04, by doing FBM flashing of: boot.img, ubuntu.img and recovery.img. Empty-Tray-Workaround is not applied.
53.304). Test Case Steps:
53.305). I get back the Factory Fresh Android 9 on X605L 4/64.
53.306). In TWRP-3.7 I restored "20180802...FreshAndroid9" backup (which I can share with you if needed).
53.307). I booted X605L to Android 9, set up Language, accepted License Agreement and all the rest, skipping what possible to save time and to simplify the step.
53.308). Now I have the Factory Fresh Android 9 on X605L 4/64, with the Stock Recovery, because TWRP has been replaced by Android at boot because Magisk was not installed.
53.309). Therefore, now we have the real life use case - the typical, fresh, not customized Android 9 on X605L 4/64 and user wants to manually flash UT20.04. BootLoader is Unlocked. Developer Mode is activated by tapping 7x the Build Number (TB-X605L_S210283_221108_ROW). In Developer Options the USB Debugging is turned on.
53.310). I connect it to PC via USB-C, I executed "$ adb devices" in the terminal on PC and I confirmed "Allow Debugging" > Always > ok > I run ubports-installer on PC ($ ubports-installer &) > "info: device detected: X605" > My device is unlocked > Ubuntu Touch > Install > 16.04 stable > Wipe userdata (mandatory on first install) > OK > rebooting to bootloader > I'm in FBM > Flashing Firmware > Cleaning up > Continue > Waiting for the device > Manually switched to Recovery > VolumeUp+Down+Power > Not disconnected USB-C > then VolumeUp+Down until Ubuntu Recovery appears > waiting some time > Pushing files message on PC's screen > Reboot > USB-C cable has been disconnected > Installing update > Reboot > Lenovo Logo > Ubuntu Touch . . . . Logo appears > booting quickly > Success > Languages/Countries list appeared > I did a quickest possible setup in order to check the System Version: Ubuntu 16.04 (OTA-25) > I powered off X605L > Switched to FastBootMode manually (VolumeDown+Power) and I connected X605L to PC via USB-C cable.
53.311). I flashed boot.img, ubuntu.img, recovery.img and I restarted in FBM:
$ fastboot flash boot boot.img; fastboot flash system ubuntu.img; fastboot flash recovery recovery.img; fastboot reboot
53.312). Lenovo Logo appears, I disconnect USB-C cable > Ubuntu Touch is booting > Welcome to Ubuntu Touch 20.04. We will make sure your device is ready to use Ubuntu Touch 20.04 > Next > Major Changes > Welcome back > Your device is ready to work > Continue > Settings > OS > Ubuntu Touch 20.04
53.313). I consider the TC to be PASSED
53.401). Name and Status: TC003 - [Not Yet Available]
53.402). Description: "Android9@X605L -> via -> UBports-Installer -> to -> UT20.04"
53.403). Details: User of an Android 9 X605L 4/64 tablet wants to install Ubuntu Touch 20.04 on the device, using "UBports-Installer".
Dear @Luksus, thank you one more time for having shared the Great fruit of your Effort and kind regards~~ Piotrek~~pe3no.