I have a Pixel 3A with LineageOS.

You can squeeze the Pixel 3 to activate a shortcut which google called Active Edge.
Lineage provides an reverse engineered implementation of that where you can set the action to one of the one shown in the image. I would like to set it to open a specific app (FossWallet) however that is not an option.

But as I do not have any Assistant App yet, I have some hopes now that there is some kind of app that acts as an "Assistant" which you can configure to your liking. eg. launching an App or some Automation. Does anybody know of an app like that? Preferable available on Fdroid.

Try dicio

Wristotle for Pebble users

Dicio is almost what im looking for. I just wish I could configure it so it directly triggers some action without having to say it. Thanks for the recommendations :)