Baud rates are matching and I still have this problem. I've read other post with similar problems but cannot figure out how to fix this.
The Bluetooth dongle works fine. Only the wired uploader has this problem.
You may try this FTDI 5V USB to TTL Serial Adapter Module for Arduino Pro Mini. (the link is just an example, you can search the keywords on other stores)
If it proves to be our uploader's problem, I'll reimburse you for the cost.
Does the wired uploader upload the sketch correctly? Do the abnormal characters randomly occur in the string?
The strange characters are usually due to baudrate but you got it right. And it still prints out some correct letters. I never see it on my end. The cause can be the synchronization of the signals.
Are you using the Bluetooth dongle or the wired uploader for the serial connection?