I wrote my own (code first) resume generator and I am very curious about your project, also for things I didn't know about like ATS. Will definitely look into it.
Amazing to hear that you are creating your own resume generator. I have learned a lot while building this project and am happy to share thoughts if you have any questions. I document my code extensively so I hope it can be helpful reference too. In regards to ATS, basically company uses ATS software to parse resume info, the most ATS friendly resume structure is the simple single column resume design since it works best with the parser by being easiest to parse.