Centroids processing tool.

However this will require a bit of prep. My recommendation is that you convert your polygons into points using the 'Extract Vertices' tool. This will give your the corners of each poly. Then select the two opposing corners and use a Point-Line tool (eg: Points to Path). Do this for each opposing vertex. Then you can use the 'Centroids' tool. This will create a new centroid between the two, which represents the midpoint.

Centroids processing tool.

However this will require a bit of prep. My recommendation is that you convert your polygons into points using the 'Extract Vertices' tool. This will give your the corners of each poly. Then select the two opposing corners and use a Point-Line tool (eg: Points to Path).

Do this for each opposing vertex.

Then you can use the 'Centroids' tool. This will create a new centroid between the two, which represents the midpoint.

Centroids processing tool.

However this will require a bit of prep. My recommendation is that you convert your polygons into points using the 'Extract Vertices' tool. This will give your the corners of each poly. Then select the two opposing corners and use the 'Centroids' tool. This will create a new centroid between the two, which represents the midpoint. Do this for each opposing vertex.

Centroids processing tool.

However this will require a bit of prep. My recommendation is that you convert your polygons into points using the 'Extract Vertices' tool. This will give your the corners of each poly. Then select the two opposing corners and use a Point-Line tool (eg: Points to Path). Do this for each opposing vertex. Then you can use the 'Centroids' tool. This will create a new centroid between the two, which represents the midpoint.