Not that the raspberry pi is particularly low end anymore, but if you do want to run matrix on low end hardware you may want to avoid synapse as it is very resource heavy.

There are a couple implementations that support bridges here

Most android apps?

The wiki page also doesn’t exist yet so I think its probably just that they took the Evince readme and changed all the links and its not actually published yet

I would worry less about Riot being the one who is gonna be decrypting your stuff and worry more about some malicious actor who manages to get access to your computer. Running anything that communicates over the internet with such high priveleges is a massive attack surface