Install pulseaudio on RaspberryPi OS, pair and connect BT speaker, download/enable autoconnect and create cronjob to play a 1Hz sound for 1s in order to keep BT Speaker always on.

rpi_bt_audio_config.sh

#!/bin/bash

echo "Installing PulseAudio and BT Module"
sudo apt update
sudo apt upgrade -y
sudo apt install -y git pulseaudio pulseaudio-module-bluetooth
systemctl --user enable --now pulseaudio.service

MAC_ADDRESS=<MAC_ADDRESS>
echo "Pairing and connecting to $MAC_ADDRESS"
coproc bluetoothctl
echo -e 'agent on\npair $MAC_ADDRESS\ntrust $MAC_ADDRESS\nconnect $MAC_ADDRESS\nexit' >&p
output=$(cat <&p)
echo $output

echo "Lowering the volume to 40% and testing speaker"
amixer set Master 40%
speaker-test -S5 -c2 -l1 -twav

echo "Setting up auto connect"
git clone https://github.com/jrouleau/bluetooth-autoconnect.git
sudo apt install -y python3 python3-dbus python3-prctl
sudo cp bluetooth-autoconnect/bluetooth-autoconnect /usr/local/bin
sudo cp bluetooth-autoconnect/bluetooth-autoconnect.service /lib/systemd/system/
sudo cp bluetooth-autoconnect/pulseaudio-bluetooth-autoconnect.service /lib/systemd/user/
sudo systemctl enable --now bluetooth-autoconnect
systemctl --user enable --now pulseaudio-bluetooth-autoconnect.service

echo "Setting up cronjob to keep BT Speaker always on"
sudo apt install -y sox
( crontab -l 2>/dev/null; echo "*/10 * * * * XDG_RUNTIME_DIR=/run/user/1000 /usr/bin/play -n synth 1 sin 1" ) | crontab -