The rfcomm process has higher CPU usage when connected.

The rfcomm utility is deprecated since Bluez 5.44. No alternative found yet, maybe look into PAN.

Stop ModemManager from interefering:

systemctl stop ModemManager
systemctl disable MomdemManager

Modify /etc/bluetooth/main.conf:

[Policy]
AutoEnable=true

Start and enable bluetooth service:

systemctl enable bluetooth
systemctl start bluetooth

Create systemd service /etc/systemd/system/rfcomm.service

[Unit]
Description=RFCOMM service
After=bluetooth.service
Requires=bluetooth.service

[Service]
ExecStart=/usr/bin/rfcomm watch hci0 1 setsid /sbin/agetty --8bits --noclear -L rfcomm0 115200 linux

[Install]
WantedBy=multi-user.target

Enable and start the rfcomm service:

systemctl enable rfcomm
systemctl start rfcomm

Stop ModemManager from interefering:

systemctl stop ModemManager
systemctl disable MomdemManager

Add user to dialout group.

On the server side:

bluetoothctl
  discoverable on
  pairable on

On the client side:

bluetoothctl
  scan on
  -wait to discover server-
  scan off
  pair <mac>

On the server:

bluetoothctl
  discoverable off
  pairable off
  trust <BT-MAC of client>

Connect to the serial console over bluetooth:

# There should be a rfcomm0 device
crw-rw----. 1 root dialout 216, 0 11 jun 08:14 /dev/rfcomm0

# If device is nonexistent, create one
sudo rfcomm connect /dev/rfcomm0 <BT server-MAC-address> 1 &
Connected /dev/rfcomm0 to <MAC> on channel 1
Press CTRL-C for hangup

# Connect to the serial console:
screen /dev/rfcomm0 115200

After resizing your terminal, run: resize.

Check bluetooth is not disabled:

rfkill list