Can second that email works great to send Epub, never had issues in years
paf
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.
I'm not 100% sure but this might also depend which OS you are running on your pi. Did change from SD card to SSD few years back for my home assistant setup (now running with a mini pc) at the time I went for a geekpi adapter (can't recall exact model).
Try a quick ducduck search "sata adapter #yourOS#" and see if a list come up. Here is one for HA (but surely more models are compatible than the ones listed here) https://community.home-assistant.io/t/working-usb-enclosures-and-adapters-with-hassos/212763
Edit: as for SSD, I went for Samsung EVO model
Sidenote: all SSD will be more reliable than a sd card because they are less sensitive to rewrite counts
I don't see why you think those are web based, usually people run this at home but glad you find what you are looking for
As far as photo librabry, you can compare the different solutions at https://github.com/meichthys/foss_photo_libraries
Relying on 4 nvme drive for low power isn't ideal as nvme drive needs about twice more power compare to sata SSD.
On your Xiaomi first time you launched app, you can determine wether you always allow or allow on prompt (but you should still be able to manage that setting afterwards). You also need to disable battery optimization for that app and set app to be able to auto launch in background. This is at least what I do with my home assistant app to determine GPS location in background (app record every 15mn) and no issues with battery draining.
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
They do have hub (probably pi like hub) with ZigBee and/or zwave but best to use a small nuc (or similar) and get a ZigBee dongle. Will be a bit more expensive but worth it. Have no idea about pricing for subscription service but I'm not sure they can even match the power of home assistant automation capabilities.
Haven't read so can't tell you but you will find info at https://leminal.space/post/11699480