I think you should plan your trip by yourself with the data from internet. Don't go with agency as they will cost you more, If you will plan a trip by yourself and contact direct to people this will help you reduce the cost.
What you can do is take help from tripoto's AI chat bot. It will help you in seeing other people's experiences, this would help you in planning a trip on your own. This would always be the cheaper option if you are smart in selecting hotels search multiple sites.(the most variable cost factor in trips) You can also search for flights on Skyscanner which would help you in getting the cheapest one and it has different features as well. Another help could be that you look into the itineraries of the travel agencies. This all should help you in planning a trip wa cheaper than the agencies. Agencies are there to earn profit as well. If you are going on a trek, it's recommended that you book from a travel agency. Trekking on your own could be dangerous and even life threatening if you don't take all things into consideration. I would recommend Indiahikes. Small groups and great facilities.