As far as I can tell, it isn't possible to lock only the attachments. I have tried to create workarounds but would like to propose a new feature to lock only the attachments.
The issues with checking out an item to lock it from being changed is that the flow can at times error out as I am setting up an Approval flow that once 'Start and wait for an approval (V2)' step has been started the properties of the attached file need to be locked from changes.