This might not answer all of your questions and I only have very small number of cameras, but I would recommend to buy any IP Camera that supports RTSP and then create a seperate VLAN or LAN for the cameras to not be able to phone home or anywhere else. I actually have a complete isolated smarthome network using a opnsense Router and it works Quite well and at least feels safe.

Another way could be to invest in unifi / ubiquiti cameras, they have a good Reputation. Integration in frigate or homeassistant I dont know.

Its always DNS

The shelly Sensors are really nice.

Finamp, very polished, actively developed and cross Platform. Requires a jellyfin instance. But thats a nice diy project in it self, can only recommend

IMHO there is no perfect solution for this, but I switched to Searxng (which can be selfhosted) and Im happy with the results. Please note that Searxng is a Meta Search engine, that basically means its using multiple other search engines with less tracking afaik…

Are the external USB drives using a external power cable or just USB power? To me it sounds like you are hitting the Pis USB Power limit. You may want to get a cheap external USB dock with dedicated power supply

I tried both (for selfhosting and features) and Immich is way better IMHO

https://selfh.st/apps/