You're probably at the mercy MB / the dealership, unfortunately. First step for you is to read, in full, your complete lease agreement, and see exactly what it says about early termination and the penalties.

Accept that that is the worst that can happen, and you might have to do just that. But go and see your dealership. See what they can offer you - but have your 'weapon' in the background, that you can terminate the lease, pay up, and go ... to a different manufacturer. Even if you've no intention of doing so, the threat gives you a little bit of strength to negotiate from.

I think it will be very expensive to get out of the lease, your car has suffered a big depreciation hit and that will reflect back. I hope Mercedes do the decent thing and soften the blow for you and you can strike a deal. Clearly whatever model you go for test drive it as much and as long as you can and avoid the harder AMG suspension.

There are specialists who can advise on correct seating and driving position coupled with a replacement orthopaedic driving seat that supports your back and legs. Could be a less expensive option and much better for your long term health.