for back muscle stretching, simply bend down your trunk, u will feel stretching feeling over your back
for ur concern of bridging, i have an opinion for you if u want to involve more buttock muscle
1. have your knees touching each other, it will involve less hamstring
2. activate your buttock muscle before doing bridging
3. think of the buttock muscle during strengthening
** actually hamstrings is also posterior tilter, strengthening of this muscle can also benefit you