[Fire HD 8 and HD 10 General] retyre:
1. Should have done anything, above, differently or in a different order, etc?

-- I would chalk it down to the "bloatware" you disabled. If you find a list of items to disable: (a) Copy each item into a command format in notepad: e.g., pm disable #1 <br> pm disable #2, etc. (b) Instead of disabling everything at once, do it a-few-at-a-time (e.g., if you have 50 items on your disable list, break it into sets of 10). (c) Reboot after disabling each set. If the device reboots w/o a problem, you know the items in that set are safe to disable. (d) Move on to the next set. Rinse and repeat. (e) After you identify the offending set, disable item-by-item and you will be able to identify the offending "bloatware." I know this is painstaking, but it's the only way to get to the bottom. If you know from experience that some are safe to disable, you can cut down some time here.

-- For launcher and lock screen issues, start by disabling the usual suspects. (From my post #23 in this thread, you will have to disable paladin in addition to firelauncher.) If that does not solve your problem, use an Xposed module (I use Lockscreen Disabler).

-- If the device is stuck on the "fire" screen, your first instinct should not be to resort to adb sideload. Most of the time, the device will be adb-accessible from a PC. Connect the fire-stuck device to your PC, run adb shell, su, enable all the items you disabled, and reboot. The device should boot back up. Once there, disable as described above.

-- Use adb sideload with extreme caution. Preloader changes in 5.6.1.0+ will result in a hard-brick (from rollback) if you aren't paying attention to the version being sideloaded.

2. Should I install the Prerooted Xposed 5.6.3.0 image?

-- Only if you want to. I haven't seen a changelog. I have been updating the OP with the latest prerooted image only because there are people who want the newest release. I am happy with 5.6.0.1.

3. If I do, will my previously installed apks get borked?

-- Usually, no. Most of your apps and settings reside in /data. Only the apps you have (manually) moved to /system will disappear.

-- In any case, you should first install FlashFire and make a backup of your current setup before you begin experimenting.

4. Do you have any other suggestions to fully optimize the system, probably to emulate yours, more or less?

-- Like I said, I am happy with my 5.6.0.1. I use the device for media consumption and hands-free Alexa. I don't think anything in 5.6.1.0+ has made things better in those areas.

-- One of the reasons I settled on 5.6.0.1 is that the preloader remained the same across 5.4.0.0/5.5.0.0/5.6.0.0/5.6.0.1. In fact, I make it a point not to save any of the update .bin files from 5.6.1.0+ on my PC (to eliminate the risk of a hard-brick from an accidental downgrade).

-- One way to optimize would be to run a boot manager to find out what's being run at boot. Disable as you see fit. Given the memory constraints, you want a "lean" system if you can manage it.

5. Last, what to you think the odds are to get the bootloader unlocked?

-- Slim to none, mostly the latter. You will not miss it if you can get FlashFire working.