For me it was KitchenOwl. It's shopping list works and looks similar to Bring, which we had used before and made the transition for my wife easier.
EarMaster
If you don't plan on supporting this for at least a few years I would say no.
If you don't expect to have a decent local userbase I would say no (local in the sense of a common interest, e.g. it might be useful to host an instance for all members of even a small company if you expect a decent amount of posts).
If it's just you and your family (let's be honest: it's just you) I would say no.
The fediverse can federate, but if everyone is using their own server it's just a fancy peer to peer network.
I don't know if Home Assistant is so niche. Everyone who does some form of smart home comes to the point where there are several manufacturers forcing you to use their own app. If you're lucky you can use something like Google Home or Siri to have a unified control interface, but these are usually very basic. You can try to stick to one system for as long as possible, but sooner or later that will fail. A system like Home Assistant is the inevitable solution to these problems and it is a very good thing that HA exists as a strong and open software to solve this problem.
I have an Intel NUC (3rd gen I think - it's several years old by now) which runs Proxmox, which runs several VMs including Home Assistant on HAOS. The only thing I did was upgrade the RAM as the VMs eat this quickly...
Other services I run on this small box are AdGuard, Paperless-ngx, KitchenOwl, tt-rss and two Nightscout instances.
While almost everyone here seems to hate AI (maybe for the wrong reason, but who am I to judge) I like to have AI as it is able to provide answers a simple search engine cannot.
What I don't see is hosting something like this myself. The managing of source and indexing them would take too much of my, my server's and the web servers to be indexed energy (maybe I am wrong).
There are already good solutions (OpenWebUI with Ollama) that can be tweaked to almost do what you're describing and the AI models get better every month, so I don't think a custom AI search engine could keep up with it.
For a general guide on how to make ssh more secure I stick to https://www.sshaudit.com/
You can check your config and they also provide step by step guides for several distros...
I think what he means is that if your backup is triggered from your main server and your main server is compromised the backups can also be attacked immediately. If the backup is requested from the backup machine you will at least have the time between the attack and the next backup to prevent the attack from reaching your backup machines.