Need to replace the freehub on my Giant anthem x1 29er

SOURCE: i get back pain with

You're asking us for a subjective answer that requires alignment with personal preference on your part to work. Not only height but hand position and width come into play.

The general geometry of a mountain bike was carefully considered to keep your center of gravity between the axles until you encounter steep up or downhill trails. Then you have to move around a little to stay low and over the bottom bracket.

Getting Riser Bars or simply flipping the stem over or getting one with a different rise and reach might give you the added height you need. If you LIKE the handlebars, the stem is the best option as most new ones come with front loaders (detachable front caps) that allow replacement without stripping components off one side of the handlebar. In minutes a Bicycle Shop could swap any number of them onto your bike for you to try.

I would recommend you get used to the position that yields a careful compromise between aerodynamics nd uprightness when needed.

I always ask people who have issues with an efficient riding position "who ties your shoes for you?", because THAT is a far more extreme bent over position than one would find on anything short of a time trial bike.