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It's nice to see so much dedication for solving a problem you'd think to be solved already.
The normal "solution" (ie what I do) for this would be using an ESP8266/32, it has 3 UARTs and I think you can bitbang some more.
I'd personally grab something like https://github.com/hopkinskong/esp32-ssh-server and hack public key auth I to it, but I can imagine hacking some old 3G modems would be a lot more fun.
How do I change the baud rate on this thing? Without recompiling and flashing the firmware?
Presumably you don't. You can probably hack something into the firmware to change the baud rate based on the port you're using to SSH in, but I haven't had any need to myself so I can't tell you.
Yeah, I was really surprised to see a USB to serial adapter used. Would have thought a couple I/O pins on the MCU and a max232 would do the trick.

I think that’s how people modded SD cards onto their WRT54Gs back in the day.

This makes me wonder what are all the uses for old electronic equipments, by repurposing some of their chips. Is there any database/resource on this?
I do not know of a database but the electronics maker/tinkering scene is something I would look into for that. If you happen to find something like that, let me know!

Here is a way to convert a video capture card into a GPIO board if you want to have a look at that.

https://hackaday.com/tag/hauppauge/

> The UART cable length limit makes it difficult to work with on a messy desk

Not sure what OP is running, but I’ve run rs232 at 115.2 over 50’ (or was it 100’?) for months at a time. Over dollar-store grade RJ-11. And one side wasn’t even sending +12/-12, but 0 and 5V.

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Difficult to work with because of the mess of cables, not because it wouldn't actually work, I think.
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> esp-link: While being very affordable, it offers no transport security.

On your local WLAN? Transferring mainly Linux kernel boot logs? Ok...

That depends on your use case, many devices also present a login or even root shell on the serial console on boot. I'd prefer my WiFi to rs232 converter to have security out of the box just for that use case.
According to the finest experts in the field (read: some HN comment I read), it is not considered best practice to trust your LAN. Defend it, yes, but always apply defense in depth. Putting all your trust in a single boundary is risky.
This is interesting because Big Sur no longer supports the USA19HS anymore.