Territorial Single Cat Advice?

Hello! My girl Molé is my only cat. We were told at the shelter she won’t do well with other cats, as she came from an animal hoarder situation. She gets VERY territorial when she sees outdoor cats in our yard. She throws herself against doors and yowls. The yowling doesn’t bother me much, but I’m worried about her physical wellbeing when she does this. Any tips on easing her territorial woes? (Also fyi she is an indoor cat with supervised/leashed outside time )