"Comfortable with" is a common phrase that is used to describe one's level of ease with something or someone. If you're looking for synonyms to replace this phrase, there are a variety of options available to you. Some suggestions could include "at ease with," "familiar with," "used to," "accustomed to," "in tune with," "compatible with," "content with," "confident in," or "adept at." Each of these phrases highlights a slightly different aspect of someone's experience or relationship with a particular thing or person, but all convey a sense of ease and familiarity. By using one of these synonyms, you can add variety and precision to your language while still maintaining the same basic meaning.