But anyone can set up an instance and become admin 😉

There should be no leaking through SSH. SSH connects directly with your targeted client. From the hub you can only see that the communication protocol is SSH but not what is transmitted.

I'm happy to help.

SAS has its own connector. You would need a mainboard or PCI card with it, special cables and drives. For SATA you should be fine with the "consumer" power and data sockets. There is a SATA sockets witch combines 4 ordinary SATA interfaces. I'm not sure how its officially called, but you can get adapters for it.

Starting with 2U you should find cases that could fit a normal ATX PSU. The problem I see with the 2U cases is, the cooling. If you stack the cases on top of each other, the air intake would be blocked. The 3U cases are high enough to flip the PSU by 90° so the air intake is from inside the case.

For smaler PSU I found the Flex ATX size. They should fit inside even a 1U case.

You can get empty server cases, I guess even in the UK. Search for server case or 19" case. These come in different heights. Starting with a height of 2 or 3 U you can insert decent sized and therefor silent fans.

For the WAR depending on your DIY abilities you could build your own closet, add some 19" rails to an existing closet or get a small 19" rack

Take a look at the documentation of Lemmy.

The server-to-server communication uses the ActivityPub protocol

I would guess the safest way would be through the project side: https://f-droid.org/

Something like FreeTube?

It has its own way of scraping YouTube and is using Invidious

You can also look into Piped

For backup you can look into rclone. It's what TrueNAS is using. You can set rclone up to only upload your data encrypted to a lot of storage providers like BackBlaze, Google, Dropbox, S3 etc. So you don't have to trust the provider with the privacy confidentiality of your data.

Instead of a Raspberry Pi you can look into used/refurbished Thin Clients which are way cheaper than a Pi at the moment.

I would strongly recommend to start experimenting in your local network too and not rent a VPS in the internet. There is a very high risk it will end up in someones botnet, if you don't know what you're doing. For your local network, make sure to not forward any ports from the internet into your LAN and disable UPnP and the like. After that you should be pretty safe from direct attacks from the internet.

For more detailed tips you should tell, what you want to self host. Start small and learn along the way.