5 days post op Breast Reduction, I just noticed this morning like a hollow/swooshing sound when I tap my breast. Any suggestion?

It certainly will take time to resolve.  It may not return to normal, but it is not possible to predict what is going to happen this early after surgery.  Be patient.

After breast reduction, you should be in your house doing nothing for a week.  After that you can go back to work.  No exercise for 1 month.  By 6 weeks most people are able to do cardio and will feel very well.  You should be able to go on holiday.

It is not really possible to change the breast for FTM without surgery.  You need to find a surgeon that specializes in the procedure and speak to them.  At your young age they will require that you speak to a counselor to help you make decisions.

You may be doing too much in terms of physical activity this soon after surgery.  If you do something that results in swelling, stop.

The dark vertical line is most likely part of your formerly large areola that weren't completely removed at the time of your original surgery.  I am not sure if tattooing would provide you with a good color match with the adjacent breast skin.  You would need to ask a tattoo artist....

In my opinion, assuming your breasts healed as expected, at 8 weeks postop from breast reduction surgery, you should be back to your baseline energy. Best to make an appointment with your primary care physician for an exam to make sure there is no underlying health problem.