Home assistant has a voice assistant but it isn’t a voice assistant. Also, you are correct it is made to control a smart home (at the moment) that also include music threw external provider (or local files) but you can’t search the internet threw it. You can either install it locally or pay for home assistant subscription which include voice assistant threw cloud.

Give it a shot, description from fdroid says it supports subtitle download.

Long time I haven’t used it but nova video player should do it

It is a good voice assistant but it isn’t an ai voice assistant, Also only works for your home assistant devices/entities unless you link it to gpt

ZigBee are not designed for high bandwidth applications but +1 for Home assistant