そろそろ and もうすぐ can both be translated as 'soon', but I feel they have different uses and connotations.
My take on it is that そろそろ is slightly more formal and has more of a meaning of 'It's about time (we)...' as in this example:
そろそろ出ましょうか？ Shall we get going? / Isn't it about time we go?
Whereas もうすぐ sounds more urgent, and has more of a meaning of 'almost' or 'just about to', like in the following example:
もうすぐ夏休みだ。 Summer vacation is just around the corner.
Can these be used interchangeably? Can I say the following sentence:
To me it sounds a bit stiff and formal.