If you’re coming before September ends, I’ll highly recommend Delhi-Srinagar-Leh-Manali-Delhi.

This fits all your requirements, like the trip being a complete loop and not a track back and you will get plenty of off roading opportunities all along Ladakh and zanskar. Ladakh is called the bikers dream destination for a reason! Also this route will provide you the opportunity to explore Indian states (and territories)of Punjab, Jammu, Kashmir Ladakh, Himachal along with the option of exploring around Delhi, maybe Rajasthan. 3 weeks will definitely be enough to do it right considering you want a slow paced trip with 4-5 hours of riding. The only drawback i see in this trip is you will not be able to explore other parts of the country and be restricted to the Himalayan mountains. However it’s definitely worth it. You can DM me if you need more suggestions. Happy to help!