I have tried hosting a Tor relay on a VPS in the past and it was bottlenecked by the CPU at barely 20MB/s, although to be fair this was without hardware AES. More importantly for you, the server's IP started getting DDoSed constantly and a whole bunch of big internet services just immediately blocked the address (the list of relay IPs is public and many things just block every address on that list instead of only exit nodes). So any of your machines are probably at least somewhat up to the task (ideally if they have hardware AES support), but this is definitely not something I'd do on my home network.

Tor operator here.

If you don’t have a second IP for your relay, don’t host at home. You will have CAPTCHAs everywhere, many sites will block you and your ISP will eventually contact you to stop degrading their IP space reputation.

Most website owners don’t discriminate between Tor exits and relays. They subscribe to block-lists that include all known Tor IP addresses. Major online services will make your browsing experience really shitty and once you’re a “known Tor IP” it will take months to remove that reputation.

You can run a Bridge instead, but you will eventually have the same problem.

A relay node should be no issue.

An exit node is what's more difficult

For the Tor operators, how to you mitigate the risks associated with what's being trafficked across your network?

I'm a Tor relay operator. For years, I ran a Tor guard/middle relay, but I often ran into issues with certain services and websites that would blacklist my IP address within days of changing it. After dealing with that for a long time, I eventually got tired of the hassle and switched to running a Tor bridge instead, to help people bypass censorship in authoritarian countries. I think it's a better fit for a home connection.

https://snowflake.torproject.org/

I feel like I should mention the Snowflake browser plug-in, which to my understanding, creates a relay in your browser basically.

Any of those can host a relay, the question is what you expect the throughput to be. I don't expect the Zero W to survive 5 sustained Mbps, but the 4B and the ThinkPad should easily eat 20 sustained Mbps.

Host a middle node

Keep in mind that none of the hardware you listed is great for hosting. It could work but I would go for somethinga little more modern. Something like a i5-7500 would be great. Get a router with vlans since you probably will want to segment the network.

For networking make sure you have a solid low latency reasonable bandwidth connection. I would do gigabit fiber since that will provide a decent amount of bandwidth while having low latency.

Also you could look into running a i2p node on the same machine since they both need volunteers.

If you do end up hosting on a VPS, consider one with renewable energy since tor nodes can consume a lot of power over time - or for home setups, you could run it off a portable power station to offset electricity costs (check out power station comparisons to find the best wh value for continous loads).

there is an official docker container to run a bridge, which is probably the easiest option. no idea if it supports pi/arm though.

Hey there are various ways to do it, but if you have a suitable bit of hardware I'd self host at home - just because! Pi 4B would be better but you can quickly get loads of traffic so if you broadband isn't great it can slow things down! If you want to host a tor relay and self host other stuff there are means and ways to do that as well. I have run a tor relay before through yunohost for example