Just facing the same problem, so I solved to use the method with 'async' and 'await' into 'onBeforeMount'.
<script setup>
import { ref, computed, onBeforeMount } from 'vue';
const valueThatIsFilledInBeforeMount = ref('')
const deffer = async () => {
valueThatIsFilledInBeforeMount.value = 1;
};
onBeforeMount(() => {
console.log("onBeforeMount")
await deffer()
});
const myComputedProperty = computed(() => {
return valueThatIsFilledInBeforeMount.value + 1;
})
</script>