Exclude internal conversation/internal email domain in a single conversation from FRT and RT calculation

We need the ability to apply and remove a tag anytime an internal email is received in one conversation and only consider for the FRT and RT calculation replies to external email (outside our organization and/or with specific email domains) . Actually if we apply a specific tag on identify internal emails in the same conversation this will exclude the entire conversation from reply time metrics, and removing the tag will then reintroduce the entire conversation back into the metrics. Unfortunately, this means that there is currently no way to exclude only internal messages while keeping the rest of the conversation included in the reply time calculations. This is a must have for our organization since our KPIs need to reflect only FRT & RT for external email (outside our organization and/or excluding specific email domains)