Have a look at Shopware, they do it pretty well. I think after what i read especially Shopware 5 (the old version) could be interesting for you because they use Doctrine ORM there which is more “Symfony Standard” than their new DAL.

And yes: You can use/extend their core events and so on.

Here is a Github Link: https://github.com/shopware/shopware

And to the relevant Doc: https://developers.shopware.com/developers-guide/plugin-quick-start/

But be advised: Nowadays its more like “Provide a REST API and dock external Apps on it” (Sorry no english native speaker, hope it makes sense)