Post-Brexit MCF and IOSS - my brain hurts

SITUATION: My company is;

• UK VAT registered
• VAT Registered in DE/ ES/ FR/ IT/ PL/ CZ
• I have inventory in Amazon UK and EU Fulfilment Centres for FBA
• I use Amazon/ Avalara to file VAT across all EU markets
• I have a UK based Shopify store

QUESTION:
Example Scenario;

• A German customer buys from my Shopify store (which is configured to calculate VAT based on customer location)
• I fulfil the order using MCF from inventory in Amazon DE fulfilment centre

CURRENT PROCESS:
I add the transaction to the ‘Additional VAT Report’ to form part of the Germany VAT filing for that period. That’s it.

DOUBTS:
Recently I have been bombarded with several opinions that IOSS possibly needs to be involved somewhere in this setup but I cannot see a reason why it should. I’m not physically sending anything across the Channel (i.e. UK > EU) to trigger a VAT collection question for the customer.

Can anyone please help me banish these opinions and reassure me that my current process is indeed correct?