This is Dallas from the Social Care team here at Shopify.
First of all, I would like to commend you on your store. It is very simple and easy to use and the layout is also very self-explanatory so I didn't get lost or anything. Yet, it is still very visually appealing, koodos.
If I am being perfectly honest it will be harder to be super successful if you are only on one platform as you are only going to reach out to one demographic. However, with that being said doing one thing really well is better than doing 4 things not well at all. Then once you have one platform going strong you can add on a second platform, so on and so forth. With that mindset let's break down what one will be best for you.
You say that Pinterest and Instagram are the best ones for your business specifically? They are both really good options and you are even able to sell on both, however, I have no way of saying what one will be better for you in the long run. For something like this it always depends on what you are selling, how you market them and so on. In my opinion, I would suggest running both of them. This way you can really figure out what one will do a better job for your product.
For my business, I use the app called Later. It is not an app run by or supported by Shopify, but as a business owner it is definitely a tool that I recommend. With Later, I am able to connect all my platforms (IG, FB, Twitter, Pinterest) in one location where I can easily schedule the posts that I want to share. Let me tell you, it has been a LIFESAVER!
If you really would only like to use one platform for now then you just need to make sure that you target properly: Age, Gender, Interest, & Location. Location is usually the feature that is overlooked. However, you really need to know who you are marketing to and where they are.
Since I am not able to make a decision for you as to what one will be better, because it is based more on personal preference, I have shared two blogs that will give you some great information for each.
These should give you some great tips on each platform and will hopefully help you to decide which one you would like to go forward with. I will also add that Later is still helpful even if you only are using one platform.
If you have any more questions please don't hesitate to let me know, I am always happy to help.