Finally we manage to root the device! Many thanks to all of your efforts.
Just refer to HappyZ's well written guide: https://github.com/HappyZ/dpt-tools
For whom have never used python like me (and probably using Windows):
(1) Install Python 3 and add it to PATH.
(2) Install MINGW64 and run scripts here instead of Powershell due to xxd issue if you are on Windows.
(2) pip httpsig pyserial on bash.
(3) Download HappyZ's dpt-tools and unzip.
(4* this issue is fixed by HappZ)
(5) Follow HappyZ's guide. You should execute dpt-tools.py in the folder you unzipped to use get-su-bin because of how the script is written.
Some suggestions after rooting (let me know if you have better ideas):
Here is my setup: install "E-ink Launcher" and "Multi action home button" using adb install.
Use adb shell am start -a android.intent.action.MAIN to change the main launcher to your launcher.
Then change the setting of Multi Action Home button (say, the height should be large to be visible in the bottom) and assign its function to be Home for click and Back for double-click.
Whenever you want to use Sony's apps (these are good for pdf markup), just push the home button to open the pop-up menu.
Otherwise, touch the Multi Action Home Button to access to other Android apps. So far I've never experience any crash.
Yet more tips:
Some complain fonts are too small after installing generic apps.
adb shell wm density 320 changes your DPI by 2 times (160 is a default value.) EDIT: I found 200 is quite enough that does not distort Sony apps too much.
My application is using "Tasker" to execute the above code when specific apps are open and execute wm density reset when the apps are closed.
The reason why we cannot change the global DPI is sadly because it makes the default apps by Sony so awkward.
Alternatively, I could successfully install Xposed to try App Settings but this app crashed.
You can also install Gboard (but it has no hide button, so prepare with virtual back button) if you need another keyboard.
Enjoy your DPT devices
Sony recently released a new digital paper device DPT-RP1, apparently using their own linux firmware but underlying on Android 5.1.1. Few weeks ago, some Chinese successfully hacked it to jailbreak for third-party apps (without changing the original firmware), but they don't share any information to sell those hacked devices. I'm willing to pay for it, but it is too risky to send my device to China so I'm trying to root it by myself.
I don't know much about this world, but I found some information that might be helpful. It uses Marvell A140 IoT Processor a.k.a. PXA1908. There are two Android smartphones (as the same version 5.1.1) with this chip - Samsung Xcover 3 and Samsung Galaxy Grand Prime. Fortunately, they have been both rooted in the past here.
Is this information really helpful to root my device? If so, is there any way to apply the previous methods to easily jailbreak DPT-RP1? I think the problem here is that it does not look like Android at all, so has no setting menu or developer tools. And not sure how to enter to the recovery mode since it only has two buttons - power/menu.
I'd appreciate any help or advice. Thanks!