I have a sonos system and a hifi system. I think there's a sonos amp now that is part of the sonos ecosystem, but requires passive speakers of your choice. Otherwise you can get a real amp and a port (i have an older connect).
The other option is just to not connect your actual hifi to your sonos. I did this for ages. Was fine.
I would point out, as someone with both, I love the sonos ecosystem and ease of use. It's great in the kitchen and hidden away in other rooms. However, if as stated, you're prepared to spend over £1k on speakers (and amplify them well) they should be head and shoulders over the Sonos. Mine certainly are.