Hello Alex.
Welcome to the forum.
For side effects concerning pm, you may experience a reduction in libido, lower sperm count, difficulty with achieving/maintaining an erection, fat redistribution to your buttocks/hip, softening of your skin and mental/emotional changes.
Breast tissue is permanent, fat redistribution to your breasts is not.
As far as using a lower dosage of pm to minimize the side-effects, it's hard to say. You'll have to overcome the testosterone barrier to initiate breast development. As to whether breast development requires a smaller pm dosage than other feminizing physical changes, I don't know.
I don't have much experience with breast creams, so I am unsure how suitable of an alternative they are.
You say you have a huge fantasy to have your own pair of breasts. Have you considered the consequences of pursuing such a fantasy? If you go down the NBE path, you may have to forego swimming in public. Growing breasts may create problems in current or future intimate relationships. When you have budding breasts, undressing in public locker rooms, undergoing medical examinations or exercising in public may result in embarrassing or awkward situations. Depending on how much development you achieve, you may even find it unpractical or painful to go braless throughout the day. If you haven't already, I recommend getting a pair of breast forms and wearing it in private for a while. If breast forms are enough to satisfy your fantasy, I would say stick with that and avoid the NBE route. However, if you still long to have your own breasts, I recommend carefully considering the risks and drawbacks before proceeding.
Even though gynecomastia isn't a rare or unusual condition, purposely growing breasts as a bio-male is, especially if you don't have any interest in transitioning. If the thought of doctors questioning you about your breast development and how you would like to stop/remove such development disturbs you, NBE probably isn't for you. You'll also have to face the reality that once you hit a certain point of development, your progress can be noticed by anyone passing by. Binding or dressing in layers isn't always practical or possible.