I've had this problem before where the icons don't work and show up as grey question marks, this should help you:
https://sodasoba1.github.io/sys-tweak/
That github user seems active on gbatemp looks like he wrote the guide and also posted templates for covers and carts
Cartridges
https://github.com/sodasoba1/NSW-custom-icon-template/tree/main/BLANK_CART
Covers
https://github.com/sodasoba1/NSW-custom-icon-template
Also what looks like custom icons but they seem small.
https://github.com/sodasoba1/NSW-Custom-Game-Icons-square
Unsure if they're useful but it might work
