Some fancy excel or python work is needed here

• Import all unique variant and their values eg Variant: Colour, Values: Black, Red, Green
• Export alll variant values with their new external ids
• Import all products and inplace of the name of the variant values, use the external ids of the variants values you just exported. This is a many2one field so you have to import it by seperating the ids with comma. See this https://www.odoo.com/documentation/16.0/applications/general/export_import_data.html.
  Import this step in the product template page not product variant page
• Export the imported products from product variant page with their external ids and import the boms with this ids
  
  hope this helps