Advice is free, so heck...I'll chime in.
It does sound like a dip tube issue. Draining your tank to remove sediment is a great maintenance habit. I do it and all kinds of stuff comes out when I do. This way the heat transfers to the water faster without the barrier of sediment to heat up first. It would save you energy and recover hot water faster.
If you want a new tank, then by all means get one. A fresh warranty is great, but another dip tube issue in 5 years is probably not covered. The effort to install a new tank is probably more than a dip tube replacement and the cost difference is obvious. A longer warranty is usually associated with having more anodes or a larger, better quality anode to protect the tank as well as a glass lining. These features do not protect the plastic dip tube from break down.
Additionally, my field and personal experience tells me that what is perceived as a 20-30 minute shower is actually more like 10-15 minutes of water use time and any additional time is drying off, brushing hair, etc. My 40 gallon tank does just fine with my family of four so if you want a larger tank than 50 gallons you may be heating more water than you actually need.
Hope this info helps you to make a decision on your next move.