About your first question

I'm not sure actually, I suppose the job will fail and it will be retried after the retry_after timeout

About your second question

Laravel Horizon provides a way to gracefully terminate the worker processes

See https://laravel.com/docs/6.x/horizon

php artisan horizon:terminate will wait until your current jobs are processed and wont pick new ones after they are processed.

What you can probably with horizon is to minimize the retry_after configuration for a specific queue only, so if jobs fails in this queue it will be available as soon as possible for other workers.

Probably, I can't imagine the whole picture though, but I hope that helps a little