If we have an exclusion $project only on meta after an $unpackBucket, we can move the $project before $unpackBucket. For example:

{$unpackBucket: {exclude: [], metaField: "myMeta"}},
{$project : {myMeta.a : 0, myMeta.b: 0}}
--->
{$project : {meta.a : 0, meta.b: 0}}
{$unpackBucket: {exclude: [], metaField: "myMeta"}},

We can do something similar when the exclusion $project is partially on meta. For example, after just the suggested optimization:

{$unpackBucket: {exclude: [], metaField: "myMeta"}},
{$project : {myMeta.a : 0, x: 0}}
--->
{$project : {meta.a : 0}}
{$unpackBucket: {exclude: [], metaField: "myMeta"}},
{$project :

}