One of the Related posts gave me the (simple) answer.
Apparently the auto value on the grid-template-rows property does exactly what I was looking for.
.grid {
display:grid;
grid-template-columns: 1fr 1.5fr 1fr;
grid-template-rows: auto auto 1fr 1fr 1fr auto auto;
grid-gap:10px;
height: calc(100vh - 10px);
}