Power outages are usually very short (few seconds max). Are fanless ups ok or should I stay away from it. The one I'm looking out at the moment is Eaton Ellipse eco 500
Didn't think about noise and that's important to me so thanks, is there anything special to look for the battery to make sure it can support my 3 devices at the same time? Like amp or other electric specifications?
You can use an helper to toggle from one to another and use an automation to trigger it (or a simple switch on your dashboard).
But if I were you, I would use a community add-on as dual AC isn't a niche, I guess it will keep being updated in future. I've been using some community add-on for a couple of years and none have been dropped til now.
And they are several option to replace extruder. Cheap one are the same compatible of ender 3 like this one at 4€
https://a.aliexpress.com/_EyXIa19
but you will have to print a raiser for the filament sensor
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.
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.
A complete and affordable setup that will merge anything you want inside a single app will be by using home assistant. But while it is much easier to setup and maintain than a few years ago, selling the house will require new owner to have a few tech knowledge. You also have to make sure that your setup is designed to be used by all family whatever there age and knowledge (and it isn't always simple)
Describing everything available that would make it a perfect place will take a very very long comment but here are some hint, look at ethernet cameras instead of wifi, ZigBee and zwave instead of wifi for most of the devices as those protocols are designed for smart homes unlike wifi, use as less as possible cloud base solution.
As a starter, you can check for temperatures/humidity sensors, contact sensors for doors/windows, automated curtains, switches, presence detection, lights, leak detectors, weather station, energy monitoring,...
Don't have particular case to recommend as the one I have has been printed but get one where you can add an SSD. SD card are not well suited for home assistant due to the amount of overwriting.
i'm willing to try to help you but to be honest, it is far from my limited knowledge... and i'm also not english native so i do sometimes struggle to understand some specifc docs...
Should have added in my post that power outage don't last more than a few seconds so ups should be fine