i already have project telegram bot like u want, its develop under python,, actually its work not using telegram bot,, because if we using Telegram Bot from botfather, the bot need to be administrator on the channel target. instead using botfather bot,, my method is inject your account that already joined to the channel (even if its private channel). and whatever the admin channel send,, you can forward to other channel,, and its can filtered too and adjustable for message format on the forward target.
and u need some vps to run the app for 24/7.