@pjs
Your current case is unsuitable. It is designed for ITX motherboards with the option of dual slot video card. You won't able to get a riser card that turns the Sam460 x16 slot into 2 slots and even if you could you'd likelyrun into mechanical problems fitting a card into the bottom slot.
If you want a slim case you'll need to get a low profile Micro ATX case.
LinkHaving done a quick search there does seem to be a few places still selling low profile Radeon RX550 & RX560 cards which should come with a low profile bracket and it should be fairly easy to get low profile sound cards ans SATA controllers.
A suitbale video card would be something like this.
Link