I've been working with the Nano BLE and before that with Adafruit's Feather BLE, both using Nordic's nRF52 platoform. Since a long time I have been also searching for an app or app development kit I could use to interact with the BLE module of the nRF52, but so far, after months of research, install and de-install countless apps and looking up Github a hundred over times I still could manage to come up with no result.
I'm pretty sure that if the by far most popular microcontroller platform the Nano choose the nRF52 for the next generation of boards, there is an ecosystem behind it. I just might have been unlucky with my search.
So here is what I'm looking for: an app which has some basic control tool like buttons, slide bars, RGB color selection, text input/output. Nothing fancy, just to start somewhere. I want my board to interact with my smart phone, as this is the big deal with the nRF52 (most of the other tasks I need I can do with an avr perfectly well).
Adafruit's BLE app works kind of, but it's too limited, you need a different code for each functionality with no means to combine them. There are tons of other BLE apps, but none of them seems to support the Nordic chipset (they mostly concentrate on ESP based boards). Even Blynk prides itself with supporting nRF52, but I did not find any evidence supporting that claim...
It is unlikely you will find an app that has a nice cute interface for your application. Most of the BLE apps are written to help you develop the software on your microcontroller, BLE module ...
They allow you to read and write to the GATT and to be flexible they need to use some general way of dealing with the data. Cute buttons, sliders and so on are specific to an application. Some applications decode some standardized UUID e.g. temperature, humidity or battery service, ...
I like EFR Connect, nRF Toolsbox and BLE Scanner on iPhone.
Now, that alone does not sound like a good explanation. Why wouldn't some hobbyist with iOS or Android knowledge write a cute app for a fixed number of UUIDs and place that into the stores? There should be a whole range of standardized BLE apps.
Have a look at the following page. This looks like you will need to pay $100 if you want to put an app into the public. There might be some additional membership fees before that.
So here is what I'm looking for: an app which has some basic control tool like buttons, slide bars, RGB color selection, text input/output. Nothing fancy, just to start somewhere. I want my board to interact with my smart phone, as this is the big deal with the nRF52 (most of the other tasks I need I can do with an avr perfectly well).
I suggest you look at MIT App Inventor or Thunkable where you can build your own basic BLE app.
There is a BLE extension which you need to add. The latest version of the extension is pretty robust / good and allows for UUID scanning, notification enable/disable, defining GATT services and characteristics etc.
With App Inventor you can add buttons, slide bars, text input/output etc. All the basic UI stuff.