We would also very much welcome this function.
Especially since we always have several filled tanks and switch between them, it quickly happens that some tanks are not used for several weeks and are then used again for a project.
This function has also been suggested several times, including some time ago.
Tank Mixing Maintenance - Feature Requests & Ideas - Formlabs Community Forum
Form 3 needs a Mix Tank operation - Feature Requests & Ideas - Formlabs Community Forum
Manual preheat / mixing control (+ fill) - Feature Requests & Ideas - Formlabs Community Forum