Get a Bluetooth keyboard work with Arch Linux

I've recently got a Rapoo E6100. This is a minimal and space saving Bluetooth 3.0 keyboard. If you pair it with Windows 10, it will remain paired after reboot, giving the possibility to use it since the login screen. After installing the Bluetooth stack on my Arch via the bluez and bluez-utils packages I thought the pairing process would be as simple as Windows if I used the KDE GUI menus for Bluetooth management. That's not true. The keyboard, once paired, will reconnect automatically just after plasmashell loaded, leaving me without keyboard during the SDDM login screen and, of course, during a non-graphical session.

As usual, i've searched help in the ArchWiki, founding this article. With that, i've succesfully reconnected my Bluetooth keyboard using the bluetoothctl utility. The next step was configuring the service for auto connection during boot. I've created the btkbd.conf and the btkbd.service files, enabling the last one with systemd. Let's give a look to the service file:

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 [Unit] Description=systemd Unit to automatically
  start a Bluetooth keyboard
  Documentation=https://wiki.archlinux.org/index.php/Bluetooth_Keyboard
  Requires=dbus-org.bluez.service After=dbus-org.bluez.service
  ConditionPathExists=/etc/btkbd.conf ConditionPathExists=/usr/bin/hcitool
  ConditionPathExists=/usr/bin/hciconfig [Service] Type=oneshot
  EnvironmentFile=/etc/btkbd.conf ExecStart=/usr/bin/hciconfig ${HCIDEVICE} up #
  ignore errors on connect, spurious problems with bt? # so start next command
  with - ExecStart=-/usr/bin/hcitool cc ${BTKBDMAC} [Install]
  WantedBy=multi-user.target

Line 13 enables the Bluetooth dongle, and line 16 connects it to the keyboard we gave the mac address in /etc/btkbd.conf. This should work flawlessly, right? Of course it doesn't. The service starts before the dbus-org.bluez.service is loaded and fails. However, if the service is started manually after login the Bluetooth keyboard works. After hours of trying figuring out what was wrong I've almost asked for a return on Amazon! The last attempt I made was with sddm disabled and involved built from scratch service:

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 [Unit] Description=systemd Unit to automatically
  start a Bluetooth keyboard [Service] Type=oneshot ExecStart=/bin/hciconfig
  hci0 up ExecStart=/bin/hcitool cc 00:11:22:33:44:55 [Install]
  WantedBy=bluetooth.target

This incredibly worked. I think the problem was that multi-user.target that needs to be reached earlier than bluetooth.target. I got rid of all the tidiness of the ArchWiki solution just to be sure that was not the problem, but I think you can use all of that just correcting WantedBy=. Currently I haven't an ArchWiki account nor a forum one, but as soon as I'll register I'll correct the article.

Last thing: I discovered that my Bluetooth dongle is CSR 8510 A10 based so expect some ramblings about hid proxy.