The above URL is the documentation link not the API URL, you have the correct API URL.
Try leaving serials in with no value:
"serials": [],
and then just adding tags.
If you import the meraki collection into postman you should be able to play around with formats and see the details on the 400 error request and then be able to just put what works into your python script.
I don't have SM devices I can reboot so I can't test myself.
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