Could you use a separate RX antenna? If you have enough room in your garden, an active antenna could avoid the worry over the effectiveness of relays, etc.
I don't operate below 160m, but have a vertically polarised active antenna mounted about 60 feet from a short vertical for 160m. Even without turning it off on TX, the ADC clipping/overload LED on my SDR doesn't light while TXing on 160m.
The circuit of the active antenna, and the dc injection/TX disconnection box, is at:
https://www.qsl.net/g4aon/pa0rdt_aa/While I use the "belt and braces" approach of a series lamp and back to back diodes myself... If you check the peak to peak Voltage across the commonly use back to back pair of diodes, while increasing the output of a signal generator feeding them, you will find a lot more signal level than you expect.
73 Dave