Typing in that order 3 lines of code :
adb devices
adb reboot bootloader
fastboot flashing unlock

Finally did the trick!

UBP took over and installed Ubuntu Touch!

UBports Installer 0.11.0 (snap)

• Environment
  • Ubuntu Core 20 linux 6.8.0-60-generic x64 NodeJS v20.18.3
• Device
  • amar_row_lte (Lenovo Tab M10 HD 2nd Gen LTE)
• Target OS
  • Ubuntu Touch

Settings

{"bootstrap":true,"partition":true,"wipe":true,"channel":"20.04/arm64/android9plus/stable"}

Log

info: Welcome to the UBports Installer version 0.11.0!
verbose: Running on: Ubuntu Core 20 linux 6.8.0-60-generic x64 NodeJS v20.18.3
verbose: Versions: {"node":"20.18.3","acorn":"8.14.0","ada":"2.9.2","ares":"1.34.4","base64":"0.5.2","brotli":"1.0.9","cjs_module_lexer":"1.4.1","cldr":"44.1","icu":"74.2","llhttp":"8.1.2","modules":"130","napi":"9","nghttp2":"1.61.0","openssl":"0.0.0","simdutf":"5.6.4","tz":"2024a","undici":"6.21.1","unicode":"15.1","uv":"1.46.0","uvwasi":"0.0.21","v8":"13.0.245.25-electron.0","zlib":"1.3.0.1-motley","electron":"33.4.11","chrome":"130.0.6723.191"}
verbose: Reading config file: undefined
command: exec: {"cmd":["heimdall","detect"],"error":{"message":"Command failed: heimdall detect\nERROR: Failed to detect compatible download-mode device.","code":1},"stderr":"ERROR: Failed to detect compatible download-mode device."}
command: exec: {"cmd":["adb","-P",5037,"kill-server"],"stderr":"cannot connect to daemon at tcp:5037: Connection refused"}
command: exec: {"cmd":["adb","-P",5037,"start-server"],"stderr":"* daemon not running; starting now at tcp:5037\n* daemon started successfully"}
command: exec: {"cmd":["heimdall","detect"],"error":{"message":"Command failed: heimdall detect\nERROR: Failed to detect compatible download-mode device.","code":1},"stderr":"ERROR: Failed to detect compatible download-mode device."}
command: exec: {"cmd":["fastboot","devices"],"stdout":"HPV41GPV\tfastboot"}
command: exec: {"cmd":["fastboot","devices"],"stdout":"HPV41GPV\tfastboot"}
command: exec: {"cmd":["fastboot","getvar","product"],"stderr":"product: amar_row_lte\nFinished. Total time: 0.000s"}
command: exec: {"cmd":["adb","-P",5037,"wait-for-any-any"],"error":{"message":"Command failed: adb -P 5037 wait-for-any-any","signal":"SIGTERM","killed":true}}
info: device detected: amar_row_lte
info: Installing Ubuntu Touch on your Lenovo Tab M10 HD 2nd Gen LTE (amar_row_lte)
info: configuring...
info: settings: {"bootstrap":true,"partition":true,"wipe":true,"channel":"20.04/arm64/android9plus/stable"}
verbose: running step {"actions":[{"adb:reboot":{"to_state":"bootloader"}}],"fallback":[{"core:user_action":{"action":"bootloader"}}]}
verbose: running adb action reboot
command: exec: {"cmd":["adb","-P",5037,"reboot","bootloader"],"error":{"message":"Command failed: adb -P 5037 reboot bootloader\nerror: no devices/emulators found","code":1},"stderr":"error: no devices/emulators found"}
debug: attempting to handle Error: no device
verbose: running core action user_action
verbose: running step {"actions":[{"fastboot:wait":null}]}
verbose: running fastboot action wait
command: exec: {"cmd":["fastboot","devices"],"stdout":"HPV41GPV\tfastboot"}
verbose: running step {"actions":[{"fastboot:wait":null}]}
verbose: running fastboot action wait
command: exec: {"cmd":["fastboot","devices"],"stdout":"HPV41GPV\tfastboot"}
verbose: running step {"actions":[{"fastboot:assert_var":{"variable":"unlocked","value":"yes"}}],"fallback":[{"core:user_action":{"action":"unlock_phone"}}]}
verbose: running fastboot action assert_var
command: exec: {"cmd":["fastboot","devices"],"stdout":"HPV41GPV\tfastboot"}
command: exec: {"cmd":["fastboot","getvar","unlocked"],"stderr":"unlocked: no\nFinished. Total time: 0.052s"}
debug: attempting to handle Error: Assertion error: expected bootloader variable unlocked to be yes but got no
verbose: running core action user_action
error: Error: uncaught exception at uncaughtException: TypeError: Cannot read properties of undefined (reading 'webContents')
stack trace: TypeError: Cannot read properties of undefined (reading 'webContents')
    at click (/snap/ubports-installer/463/app/resources/app.asar/src/lib/menuManager.js:106:37)
    at MenuItem.click (node:electron/js2c/browser_init:2:35295)
    at a._executeCommand (node:electron/js2c/browser_init:2:40710)

It still gets stuck saying "Waiting for device - Asserting unlocked bootloader variable" while the tablet says "=> FASTBOOT mode..."

But it's useless because there is no menu. There is just the text "=> FASTBOOT mode..." written in the bottom left corner of the screen. I cannot go anywhere from there. The "sound up" / "sound down" buttons don't make me scroll through any options, touching the screen does nothing.

You already mentioned booting into fastboot, but have you tried doing that then starting from the start with the installer?

In Settings :
I added "USB debugging (Debug mode when USB is connected)"
I cancelled "Verify apps over USB (Check apps installed via ADB/ADT for harmful behavior)"
I cancelled "Verify bytecode of debuggable apps (Allow ART to verify bytecode for debuggable apps)"

This didn't solve the current problem, but it solved the previous one : the tablet model not being detected.

I closed and reopened UBports Installer
Now it recognises the tablet, it says :
Lenovo Tab M10 HD Gen LTE (amar_row_lte)

In UBports Installer Tools / Developper tools :
"The installation stopped due to a problem (Oh, really?). You can choose to ignore this, or re-start the installation process.
If this continues to happen, please check if you are affected by a known bug. (known bug link not functionning)
Error: uncaught exception at uncaughtException: TypeError: Cannot read properties of undefined (reading 'webContents')

Well, I guess UBP can't check whether I really unlocked the bootloader, so it just hangs.