To record multiple inputs simultaneously, you’ll indeed need an audio interface with multiple inputs.
What you are doing now is summing all the signals of your instruments together to form 1 signal and record that. That’s fine, but there is no way to split them up into separate tracks again in Cubase, for separate processing.
However, you can of course still do a multitrack recording if you do 1 instrument at the time.
Record your amp first, and then on a second pass record the mic to a different audio track in Cubase.