The software support is incredible IMHO, it's a huge reason to use these chips. I made some toy temperature sensors with an esp32 last year, they make it so easy: https://github.com/jcalvinowens/tempsensor
How do you make the step from breadboard dev to something manufacturable?
If you have done all the circuitry want to just print/assemble your own PCBs, sites like PCB unlimited will make up short runs or Digikey will handle larger scales.
Once I'd proved it worked, I pasted that 1"x1" layout into a larger footprint, and added the sensor, power supplies, and batteries. Again, I had no real way to test any of the new stuff: I just iterated until I stopped finding problems to fix, then had them manufactured. A big part of the fun of this has been having to commit to a design without the ability to test: it really makes you think. I also enjoy the exercise of writing as much of the firmware as I can while the hardware is in the mail, then seeing how much actually works when it shows up.
In terms of bad decisions... I used builtin gpio pull-up resistors for I2C: it works, but the margin is very tight, it's just not worth it (and also means I can't put the ESP32 in sleep mode in some cases...). Wifi uses phase to encode information, so having no RF matching will impact its performance beyond the -6dB I mentioned in the README. The inductor/capacitor values are much larger than necessary. The routing of the I2C lines taking a huge bite out of the ground plane under the switcher IC is dubious. Using 1.5V alkaline batteries is nice because I don't have to worry about burning my house down... but I've gone through 200+ AAA batteries over the last year, and it feels very wasteful.
I learned most of what little I know about PCB design from this youtube channel, I can't recommend it enough: https://www.youtube.com/@PhilsLab
As you'd probably guess, the fixed cost of the manufacturing was extremely high. Unfortunately I didn't write the numbers down... but going from memory, ordering 5 instead of 30 would have only reduced the total cost by ~20%. I remember a weird valley in cost-per-unit at a quantity of 30: my understanding is that JLC combines small orders, so my guess is that 30 of that board was the largest order they were willing to squeeze onto the same panel as another one.