Inclusions require more care
Inclusions require more care in firing – they expand and contract at radically different rates.
The soak at 300F is strange to me, It does not seem to do anything. Instead use your initial ramp rate up to 650C or so with a soak of say 20 mins, since it is small. Your rate of 800F (ca 400C) is liable to promote bubble formation, as it gets the surface of the glass hot very quickly and the bottom glass is still relatively cold. Slow down. Keeping a bright surface on brass is difficult. In addition to the suggestions already made, you could coat the metal in clear powder – compatible with the glass you are using of course.