I can’t help with making fireworks or modifying them to change flight duration.
Use only commercially manufactured fireworks exactly as labeled by the manufacturer.
Follow all local laws, permits, and safety regulations.
Read and follow the product instructions before use.
Keep fireworks away from people, pets, buildings, and dry vegetation.
Light fireworks outdoors in a clear area with a stable launch setup.
Never alter, disassemble, or combine fireworks components.
If you need longer aerial effects, buy a legal product designed for that purpose from a licensed vendor.
Contact a licensed pyrotechnician for safe, lawful event design and setup.