No need to worry.
Your breasts are primarily made up of adipose tissue rather than muscle, so strength training won’t significantly affect them. In fact, stronger pectoral muscles and the connective tissue underneath can help slow the sagging process as you age.
The only factor that would reduce breast tissue is losing body fat. If you want to avoid that, find your maintenance calories based on your new activity level and aim to maintain that, or consider a slight surplus if you want to build muscle.
As long as you’re healthy, focus on multi-joint compound exercises for 5-6 reps. If you need help with your form at first, consider working with a personal trainer.
A simple routine could include bench presses, overhead presses, rows, and pulldowns. Ideally, alternate these with lower body workouts and any cardio or mobility exercises you incorporate.