alexa_media_player VS haaska

Sonos is a bit spendy, so my solution is Home Assistant + Alexa media player + turning off the Echo mics + 3D-printed caps to hide the red light rings

Yup! There's a community integration you can install called Alexa Media Player. The easiest way to install it is by using HACS.

When you use https://www.home-assistant.io as your Home automation hub, you can integrate it with just about anything. For example, Alexa integration is via https://www.home-assistant.io/integrations/alexa/ and having Alexa speak through automations is accomplished by using HomeAssistant Add-on https://github.com/custom-components/alexa_media_player and you can integrate myq hubs to it via https://www.home-assistant.io/integrations/myq/ - Then via automations you setup you can have Alexa speak (announce) when doors are opened, closed, etc. for any state change. You can also use the Alexa integration to have Alexa voice routines invoke scenes which are tied to Home assistant scripts, etc.. to open, close garage doors and so much more. HomeAssistant is all about integrations, see https://www.home-assistant.io/integrations/

the ha hacs alexa media player integration provides alexa text-to-speech, works great.

I've just re-done the whole setup process from scratch, this time using this method, but it's the exact same result - the sensor is recognised as a contact sensor in the Alexa app, but it can't be chosen as a routine trigger. I've also set it up through the North American endpoint, with the same end result...

For example I use haaska to connect Alexa to my HA instance without the need for a cloud bridge service like Nabu Casa. This is not a malicious add on, it just exposes HomeAssistant commands as a REST API that is easy for Alexa/AWS Lambda to interface with. But when I open my instance up to the internet anyone can find my HA instance and start sending bogus commands to that API to try and trick it into leaking data.