Does prayer become invalid by reciting aloud in a silent prayer due to not reflecting on Al-Fatiha, as the worshiper suffers from GERD, and does this necessitate reciting silently to avoid sin?

Raising and lowering the voice in prayer are recommended (mustahabb) acts, not obligatory (wajib). It is recommended to lower the voice in silent prayers to the extent that one can hear oneself. Raising the voice does not invalidate the prayer, but it is considered a deviation from the Sunnah. Your occasional lack of contemplation of what you recite does not affect the validity of the prayer, as devotion (khushu') is recommended and not obligatory according to the majority of scholars. The advice is to strive to be present in heart, while lowering the voice in its proper place, and to seek assistance from Allah the Almighty.