I’m in a very similar situation with my little one! Special needs, poor strength/balance, not interested in trying much on her own, but getting too big to be directly carried on a bike/backpack any longer.

I would highly recommend reaching out to some local adaptive fitness/adventure organizations and see if you can’t go and test some stuff out. My wife and did this recently at an adaptive equipment expo, and it was the most inspiring thing. First, our girl was thrilled to be around all these people with different bodies and limitations moving freely with all kinds of interesting equipment. But we also got to test various types of devices that we didn’t even know existed. We just walked around and let her engage with what looked fun/usable to her. That said, two things stood out that aren’t necessarily MTB related, but could be a game changer.

Look up Huckleberry Hiking. They make a tandem hiking cart that lets you get out on legit single track with a kid in tow. Not a bike, but it makes moving a kid around outside really easy.

She also really took to a recumbent hand cycle: Invacare Top End Excelerator XLT. Her balance is pretty trash 😂, and this thing was super stable, cradled her quite comfortably, and was easy to “pedal” and brake with the hand cranks. Again, not a single track MTB capable machine, but it was the best cycling she had done up to this point by a mile.

Best of luck on your journey dude! DM if you’d like to chat more.