Here are a selection of homes Charles has in Wales and Cornwall, although I think they are all owned by the Duchy. I think William should put his foot down as Landlord and tell his father he does not have permission to lend out any of his properties to H&M but he will probably have to take the high road in the interests of family harmony.
Harewood Park is the estate in Herefordshire which he acquired in 2000 via the Duchy. It was once mooted as a place for W&C and I think it was also suggested it might suit Harry but it has been kept as "a rural regeneration project". I'm not sure a "big" house has been rebuilt as once suggested, but there are other houses/barn conversions on the property.
https://en.wikipedia.org/wiki/Harewood_Park
Llwynywermod was his hideaway in the Brecon Beacons, but he gave up the lease back to the Duchy when he became King, recognising he wouldn't have time to go there much in the future. I believe it is available as a holiday let.
www.houseandgarden.co.uk/gallery/prince-charles-welsh-home
https://en.wikipedia.org/wiki/Llwynywermod#:~:text=)%2C%20%5B1%5D%20also%20known%20as,National%20Park%20in%20Carmarthenshire%2C%20Wales.
King Charles III and Camilla have made Restormel Manor – nestled in the heart of the Fowey Valley their official Cornish residence when they visit the county.
"Such is the house’s place at the heart of the community that, when King Charles and the Queen consort are not in residence, the manor – which has both an indoor pool and a tennis court in its grounds – is available to rent as part of the Duchy Estate."
https://www.houseandgarden.co.uk/gallery/prince-charles-house-cornwall
I note interior design in most of these places has been the responsibility of Annabel Elliot, Camilla's younger sister.
Probably none of them are grand enough for the Sussexes though! 😂