How to force Siri to use ONLY internal microphone
This is an often-asked question on many sites.  Unfortunately, all those sites say the same thing: this isn't possible.

As long as you're connected to a Bluetooth device that has a microphone, the iPhone will use that mic for Siri unless you specifically change it (like you mentioned) each time you use it.  I think your best option is use a Bluetooth device that doesn't have a mic.  No mic, no problem.

I don't know if this is part of the Bluetooth specifications and can't be changed or if it's an Apple-controlled thing.  I would recommend providing feedback to Apple and letting them know this is something you want.