All me 
nothing but a 20-year-old college CS student with 0 prior experience designing PCBs. I just opened KiCad and tried to figure it out 
I have a lot of Arduino experience, and I’ve spent a lot of time watching various electrical engineering YouTube channels, which has helped immensely. Over the last few years I’ve picked up a lot of knowledge, just had never applied it to anything. Besides the voltage regulator issue, everything seemed to go pretty much to plan with the board (other than a few bodges as well).
All of the software I used is free and open source (mainly KiCad + the arduino utils), available for anyone to download.
So far the greatest barrier to rapid iteration has just been cost more than anything else. With each new batch of boards costing about $23 from China, and the BOM of each board being over $50 at the moment, costs add up quick. I also had to buy a considerable amount of tools the first time around. Needed a good ISP programmer, crimpers, better wire strippers, etc. It’s not like I’m super poor or anything, but I don’t have the money to constantly put in to this sadly. Almost all of my money from the last Christmas went into the first revision.
Because of this, I think the next revision might be the final one, so it’s gonna take a lot of time to make sure it’s 100% perfect before I order the parts. Thankfully I’ve got a list of things to fix from the first revision, I learned a lot from it.
I’m fairly busy with school at the moment too, so I might not be able to get much done til May or so, when we go on summer break.