Mixed feeding is not “halfway to giving up the breast”. If top-ups are organised carefully, lactation continues: milk is produced in response to stimulation, so the breast needs it regularly.
In practice: breast first, formula after; the volume of a top-up is chosen by age and by how much the baby actually takes; the bottle is given slowly, with pauses, ideally with a slow flow. Night feeds are best kept at the breast — they support milk production.
If top-ups are temporary, their volume is reduced gradually while watching nappies and weight. It is better to agree any changes with a doctor or consultant: abrupt decisions cost more here than careful steps.