Devices on tasmota firmware can link directly to each other using device group without setting up any automation/hub.

Can’t find any shops selling preflashed ones for the switch in UK (online at least)

Budget wise there’s:

1. Localbytes plug

2. Shelly Plus mini

Does the outhouse still have wifi coverage?

I’d recommend Sonoff zbmini L2

Yep, like others mentioned it’s innovelli

Another way to get any neutral required switches working without neutral is this small hack

As for sliders, that’s gonna be pretty hard to get since the ones that do have it are connected to tuya mcu so you can’t decouple the relay on those even with esphome. Except paddle/button dimmer like martin jerry and tessan.

Which smart switch are you using now? You can probably modify the firmware to decouple the relay