そろそろ and まもなく: approaching time to act and an event soon
Compare a suggestion to head back with an imminent train arrival announcement, without assigning either expression a fixed minute count.
The time for an action is approaching
そろそろ can indicate that it is nearly time to act. そろそろ帰りましょう。 suggests that it is about time to head back.
そろそろ準備を始めます。 says that the person will start preparing soon. It does not say that all preparations have been completed.
Announcing something expected soon
まもなく means soon. まもなく列車が到着します。 announces that the train will arrive soon.
この催しはまもなく終了します。 says that this event will end soon. It is distinct from a report that the event has already ended.
Intentions, expectations, and actual completion
そろそろ出発したいですが、まだ準備が終わっていません。 says that the person would like to depart soon but preparations are not yet finished. A wish to depart is not an actual departure.
Both そろそろ and まもなく concern a near time without fixing a number of minutes. Read any specific wording such as 十分後 alongside them.
Does まもなく列車が到着します。 report that the train has already arrived?
Show the answer and why
No. It announces an expected arrival soon. Actual arrival requires separate confirmation.