The way I see it is that you could either switch to hierarchal single tax, do the “hacky” solution you have set up or switch so that your client sets the models on the make term instead.. It will require them to go into the make term after the model term has been created but will allow you to optimize the code since then you can just loop through each make and spit our it’s relational models.
OR go your own route with a custom SQL 🙂 But honestly SQL isn’t my strong suite..
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