Never mind, I can see now that the pricing options weren’t actually under the order in my request. Fixed it so it’s like this, and it works! Amazing what a night of sleep will do for one’s brain. 
{
"idempotency_key": "1515ee78-3c3a-11ec-80e3-5bde92da8645",
"order": {
"location_id": "T753NGKW8NMQ2",
"line_items": [
{
"quantity": "1",
"catalog_object_id": "KW3IOUQMDNS7EDPWHQZBGFPN",
"modifiers": [
{
"catalog_object_id": "ORGMYD5RS7XMWAM2T56LTMWY"
}
]
}
],
"discounts": [],
"pricing_options": {
"auto_apply_taxes": true
}
},
}