T5s last longer than t8s
T8s= change every 6months
T5s=change every 12months
Even though they will still look the same way light wise after these times they will not be producing the same levels of uvb anymore and your dragon will not be getting enough uvb after that amount of time
As far as the basking bulb - I’m actually unsure. I presume it’s just when it’s no longer giving you good temps and the basking spot temp is not where it should be anymore
I was wondering the same thjg last night as I haven’t had to face that yet (I’m on my first dragon)
With my former dragon, I always used a MVB - I'm new to this t5 and basking bulb world, but love being able to use a thermostat! For basking, you are just looking for heat production - your t5 will cover the full spectrum light component. So, as long as it is producing visible light and heat, you should he good.