We use proxy account / credentials and assign this account to the Run As property on a job step. The job does not have to be run under the proxy account (only the job step). I don't agree (or maybe understand) why a separate job executor account is used to create SSIS packages. We have a small team of people who create packages using VS (Pro version) and the jobs can be owned by them or any other account appropriate for the job (may need sa or SQL Agent account if CmdExec type job step is needed). The team has the appropriate SQL Agent roles.

We also use the proxy account when provisioning file share access appropriate for our job steps. I prefer the file system for storing SSIS packages (have done it both ways and think management, visibility and deployment is easier).