Why do think henna doesn't last long? Its permanent. If you have greys, as your hair grows, you'll have to reapply just like with any other hair color. I love henna. Also, why do you think you'd have to change your regular wash/condition regimen if you use henna? I normally shampoo my hair and then apply my henna concoction for a deep condition/hair color. I leave it on for an hour or two and then jump in the shower and rinse it out. I use NTM conditioner as a final rinse and my hair is already detangled, silky and shining. I get out of the shower put my leave in with my fingers and then airdry and that's it. I'm not telling you to use henna but I just wanted to let you know that you really don't have to do anything much different from what you would do on any other wash day. I don't even deep condition after using henna because I always put a moisturizing conditioner and some essential oils in my henna mix so I'm killing two birds with one stone.
P.S. Your hair is just beautiful so ya probably should stick to what's already working for ya.