Surge protection

I have no spare ways in my CU and want to fit a surge protection device. I guess that something like the Hager VA4T2SDSPD or Wylex REC2SPD would do the needful, but would this replace the main isolating switch or would it be added in parallel like a workshop CU? Or can it be run with a smaller cable, say 10mm²?

The use case is a house packed with electronics including a Loxone smart home system.

Thanks