Very nice write up. I too am a Shopify app developer with 4 apps in the App Store, currently doing around $56k/mo and growing ~$2k/mo for the last 2 years.
Biggest thing is that the ramp up time is very slow. You're not going to get a bombastic number of installs starting off, no matter how much marketing you purchase. It's a slow climb up and the best time to launch your app is yesterday. Plan on one to two years to see decent revenue, and then hopefully continue to grow it for the next few years after that.
Shopify Plus customers and traffic spikes are definitely a big concern, especially when it comes to your pricing model. Everytime I've come up with a "worst case" scenario it has happened so you need to always have built-in contingencies around that. Whether that means reaching out to the store to create a unique plan/pricing for them and possibly adding more value services. For example, one of our apps gets between 25k to 100k webhooks per hour and the spikes are unpredictable. Our app also has a queue system to process jobs in the background which can mean there are anywhere from a thousand tasks in queue, or 500k. (Shopify API is slow and limiting for big, heavy tasks).
If you are also an actual Shopify developer (theme or private app developer) there is a lot of opportunities for upsells depending on what your app does. It seems a lot of app developers in the App Store simply have apps out there to get leads for development (service) work.
Overall, developing on Shopify has been really good. There are some annoyances with their REST API vs. GraphQL API, their documentation, their support, etc... But generally it's been smooth sailing for the past few years. But we've had very, very slow growth and only in the past couple years have paid more attention on trying to maximize our business around Shopify apps.
Our highest growing app was launched over 3 years ago so there wasn't much competition in that specific space. Lately we have had a lot of copy cats (even taking the exact same name).
Our goal is always to be improving the apps and adding features that the copy cats just can't do. We invest a significant amount of time in improving the apps.
No doubt we have lost some market share to the competition but it only motivates us more to keep on improving.
Generally it comes down to building things better and easier to use than the competition. It would definitely be very hard to compete against an app that's Free or $5/mo and has 500 5-star reviews and isn't a dumpster fire.
I tend to look at creating apps that are more complicated than what the average joe could build. One of our apps is an automation app (like Shopify Flow but 10x more useful). It's so stupidly complicated that nobody in their right mind would compete with it in a short time period. To match our features would take at least a year.
Generally we have found that all of the low-hanging fruit and quick ideas have been taken. But the more specific and complicated problems are still worth solving, even if there's one or two other apps out there that are similar.
Definitely looking for ideas with the least amount of competition is good, and really trying to figure out what kind of differentiator you can bring to the table. Usually there's at least one angle to compete on, whether it's price, simplicity (ease-of-use HUGE), hands-on support, features, beautiful UI, responsiveness, integrations...
Sometimes its hard to find the right thing to build if you don't know enough about store owner problems. There are _entire verticals_ that have unique problems that are hard to "know" about unless they tell you. Lots of different types of stores exist on Shopify. People sell classes, events, digital goods, bookings, AirBnB style stuff, equipment rentals, services (hourly based).. There are probably some available ideas that are geared more to those specific verticals.
Biggest thing is that the ramp up time is very slow. You're not going to get a bombastic number of installs starting off, no matter how much marketing you purchase. It's a slow climb up and the best time to launch your app is yesterday. Plan on one to two years to see decent revenue, and then hopefully continue to grow it for the next few years after that.
Shopify Plus customers and traffic spikes are definitely a big concern, especially when it comes to your pricing model. Everytime I've come up with a "worst case" scenario it has happened so you need to always have built-in contingencies around that. Whether that means reaching out to the store to create a unique plan/pricing for them and possibly adding more value services. For example, one of our apps gets between 25k to 100k webhooks per hour and the spikes are unpredictable. Our app also has a queue system to process jobs in the background which can mean there are anywhere from a thousand tasks in queue, or 500k. (Shopify API is slow and limiting for big, heavy tasks).
If you are also an actual Shopify developer (theme or private app developer) there is a lot of opportunities for upsells depending on what your app does. It seems a lot of app developers in the App Store simply have apps out there to get leads for development (service) work.
Overall, developing on Shopify has been really good. There are some annoyances with their REST API vs. GraphQL API, their documentation, their support, etc... But generally it's been smooth sailing for the past few years. But we've had very, very slow growth and only in the past couple years have paid more attention on trying to maximize our business around Shopify apps.