Use swoole and hyperf framework and a long running PHP application instead of symfony

Imo using swoole and hyperf framework with PHP is a much better fit for this project than symfony and apache/nginx.

hyperf: Hyperf
swoole: GitHub - swoole/swoole-src: 🚀 Coroutine-based concurrency library for PHP

Instead of running a separate docker container to just add crons and workers - you can spin up a side PHP processes from code, all managed by swoole process manager:

Or it’s also possible to setup second level crons using crons package - again all from PHP code:

Same with just task workers - you could specify how many task workers you want on application start and they would just do whatever work you need on application start - all in PHP. All without blocking the web UI workers.
mautic could then be run as a single container instead of 3+, user adoption would become much simpler.

non blocking io - send absolute maximum of emails per second.
It will have much much better performance compared to fpm or mod_php.

And throw in built-in websockets support - you could build a way better responsive UI with that: Hyperf

Imagine seeing how emails counts are flying out in real-time. would be neat.

Some large platforms written in PHP use it: https://appwrite.io/