The English translations of "もうすぐ/Mosugu" and "そろそろ/Sorsoro" are often interchangeable in much the same way, making it easy to get confused. Both もうすぐ and そろそろ represent “near future”. The dictionary definition of そろそろ is a situation that something is becoming a certain time and state. もうすぐ is representing the timelessness.
Both represent things that are expected are about to occur or be completed in the near future. Here are some examples of もうすぐ and そろそろ that are interchangeable.
もうすぐ帰るよ/もうすぐかえるよ/Mosugu kaeruyo/I will be home soon
そろそろ帰るよ/そろそろかえるよ/Sorosoro kaeruyo/I will be home soon
もうすぐご飯が出来るよ/もうすぐごはんができるよ/Mosugu gohan ga dekiruyo/The foods will be ready soon
そろそろご飯が出来るよ/そろそろごはんができるよ/Sorosoro gohan ga dekiruyo/The foods will be ready soon
もうすぐ学校につく/もうすぐがっこうにつく/Mosugu gakko ni tsuku/I will arrive at the school soon
そろそろ学校につく/そろそろがっこうにつく/Sorosoro gakko ni tsuku/I will arrive at the school soon
As I explained earlier, the sentences with もうすぐ and そろそろ can be translated into the same sentence with both words having meanings of “soon”. There is almost no difference between them and are almost entirely interchangeable. In the case of the above example sentences, もうすぐ and そろそろ mean the same thing. However, there is a slight difference. もうすぐ is a combined word of もう/Mo and すぐ/Sugu. すぐ means soon and もう is emphasizing, meaning that もうすぐ is more urgent in time than すぐ. In the same way, you can also say そろそろ with emphasis “もう”, so もうすぐ is more urgent in time than そろそろ. もうすぐ/もうそろそろ are more urgent because they have emphasis もう added to the front. From the above examples, ones with もうすぐ imply that action is done faster than そろそろ but usually we use it without caring which one is faster, it doesn’t really matter.
そろそろ has also the meaning of “quick”. When you are urging or soliciting someone to say, "Let's do something," そろそろ can be replaced with the word “早く/はやく/Hayaku/Quick”. This is because そろそろ has the meaning of “it’s about time” and represent the situation that something has become a certain time and state. もうすぐ doesn’t have such meaning so in this usage, it’s not interchangeable.
夜だからそろそろ帰ろうよ/よるだからそろそろかえろうよ/Yoru dakara sorosoro kaeroyo/It's night, we should go home now
夜だからもうすぐ帰ろうよ/よるだからもうすぐかえろうよ/Yoru dakara mosugu kaeroyo/It's night, so let's go home soon!
そろそろ終わりにしよう/そろそろおわりにしよう/Sorosoro owari ni shiyo/It's time to end it.
もうすぐ終わりにしよう/もうすぐおわりにしよう/Mosugu owari ni shiyo/Let's finish soon
Those are examples that are appropriate with そろそろ. When you change そろそろ to もうすぐ, the sentence kind of lost its meanings and sound uncomfortable. Both もうすぐ and そろそろ represent the near future, and when used in this context, they are interchangeable. But そろそろ also has the meaning of “let’s do something (soon/quickly)” and in this usage, it can’t be replaced with もうすぐ.