Looking for Advice on Achieving a More Comfortable Upright Riding Position

I’m trying to find a more comfortable, upright seating position on my bike, and I could really use some advice. I prefer to sit upright to bike anywhere, unless I'm in an area where I have to take an aggressive stance to control the bike.

I went to a bike shop out of town for suggestions, but unfortunately, they weren’t much help. I showed up close to closing time, so I got the feeling the staff just wanted me out of the store.

Here’s the issue: My current setup forces me into a forward-leaning position, which isn’t comfortable for me right now. I don't have any core strength, so I have to lean on the handlebars to support myself

I’m still working on building up my strength and endurance, so it’s not ideal for long rides.

When I raise my seat to it's max height , my arms also feel too stretched out. You can see my attempt to draw my posture in the second picture.

Most of my riding is on dirt roads, and my goal is to eventually build up enough stamina to bike for hours and cover long distances comfortably.

Does anyone have suggestions for adjustments or modifications that could help me achieve a more upright riding position? I have this issue with this bike and another MTB I have.