Eureka!

2 issues to note here for newbies such as myself:

1. always double check during aggregation and groupby methods that you still have a geopandas dataframe instead of a pandas dataframe
2. geopandas cannot accept datetime objects.

Adding this to the top fixed it:

agg_gdf = gpd.GeoDataFrame(agg_gdf, geometry='geometry')
agg_gdf.crs = "EPSG:4326"
agg_gdf.date = agg_gdf.date.astype(str)