Some mbiras do have softer keys than others. It helps to grow your thumb and forefinger nails at least as long as the end of your thumbs/finger, and play in a position that uses both the flesh and nail. Calluses help, but even in Zimbabwe many mbira players use wire picks on at least one thumb or finger. We sell one type at https://mbira.org/store/mbira-accessories/thumbpicks/. Contact us again if you want a recommendation to help you buy an mbira with softer keys.