I just tried and I think they are limitations on numerous android OS.

It isn't exactly a widget but what I have been able to do is create a shortcut when long pressing home assistant app icon on my phone

To do so, go to home assistant app settings/compagnon app/shortcut Give it a name and description, not too short according to documentation (see below), next add under dahsboard

/lovelace/home?conversation=1

https://companion.home-assistant.io/docs/integrations/android-shortcuts/

Edit: added another screenshot from settings (sorry in french but that should help). This was just a quick test, this is why I named it assist_widget. You can also change icon type

Hey man. I have no experience with your phone or oxygenos, but I can tell you that's where I found the widget. Also, I replaced my Google assistant with it, so I can long-press the circle and get the HA Assist. (You can map it to the power button, too, I think)

I'm on a Pixel 6a, so my layout may be different than yours:

Hope someone can help more!