I'm not sure if this is the best or really correct way of doing this, but it does work (at least on my setup).

Usually, I have my shelf speakers on when I'm working or doing anything on my computer. From time to time I might have to join an online meeting and there I want to use my headset + webcam, the problem is that usually the webcam is unplugged and I do plug it on my keyboard for those meetings, but then the system automatically changes the default input source to the webcam mic, which I don't want.

The aliases below allow for easily switching between my speakers or headset, while correctly setting the input source to the headset mic.

alias speakers='pactl set-card-profile HEADSET_CARD_NAME off &&
  pactl set-card-profile HEADSET_CARD_NAME INPUT_PROFILE &&
  pactl set-card-profile SPEAKERS_CARD_NAME OUTPUT_PROFILE &&
  pactl set-default-sink SPEAKERS_SINK_NAME'

alias headset='pactl set-card-profile HEADSET_CARD_NAME off &&
  pactl set-card-profile HEADSET_CARD_NAME OUTPUT_INPUT_PROFILE &&
  pactl set-default-sink HEADSET_SINK_NAME'

# To list available sinks:
pactl list sinks short

# To list available cards and profiles:
pactl list cards

For some reason, unknown to me, it is required to first change a __card profile__ to OFF before changing the input, or else it won't override the webcam mic as the input source.