Moving from Dev Kits to Custom PCB/Hardware

I have developed a prototype that uses the nRF52840 Adafruit Feather Express, a few sensors, and some other components. I am somewhat new to hardware engineering; however, I was able to get some schematics designed and I have a custom PCB that uses the Feather Express. I currently use the Arduino IDE to do development, and I’d like to stay there.

Having said that, I am trying to reduce some of my costs in my prototype. Purchasing pre-made dev kits and components is very easy to get prototyping, but when I go to Mouser.com and see the raw components the price difference is fairly significant. For instance, the raw nRF52840 is $7 whereas the Adafruit Express is $25. I have been using my prototype for several weeks, and I’ve got some interest from other people. What are the next steps to go from dev kits to a more targeted/customized PCB? I do all of the soldering myself; however, I was looking at the raw nRF52840 chip, and I’m not really sure how to solder something like that (maybe the USB Dongle is a better choice).

I know this post is long-winded. I’ve attached a picture of one of my prototypes to get an idea of where I’m at.