Can I achieve desired breast tissue growth by taking Proscar and Spiro only?

No. Although blocking the T will result in some breast growth, without additional estrogen growth will be limited as your endogenous estrogen levels are low. Estrogen as part of HRT boosts your estrogen to levels that will maximize growth.

Complicating factors are:

Age

Individual response to hormones

Genetics

The complicating factors usually prevent TS/TG woman from getting the full development that natal women get.

Hope this helps.

edit: Please don’t self-medicate, I hope you’re seeing a doctor about this.