I've had success using Airtable with an email flow for an applicant tracking system. I:

-posted a job ad with a specific reply-to email address

-every time an applicant wrote to that email address, a row would automatically be added in an Airtable base

-I would review their application and if I wanted to pass them on to a next step, I would select a pre-defined drop-down value in their row in the Airtable base to mark them as something like "Go" or "No go".

-This would trigger Zapier to send an email to the applicant with instructions on how to take our screening questionnaire in SurveyGizmo

-Depending on the results of the questionnaire, I would select another drop-down value, triggering another email, etc.

It wasn't perfect but it did help me to automate what had been a very painful process.

This sounds like a very interesting idea as a cohesive product. A normal user wouldn't be able to set this up themselves.

Could be. I haven’t researched it but my impression is that off the shelf ATS is quite expensive.