Good that you're managing to get out on a bike though.
Hi as previously mentioned I have a Motus. I deliberately wanted the step through version , for reasons similar to the last poster. Throwing a leg over a high saddle has always struck me as unsafe in traffic and anyway I don't see any particular reason for the crossbar. The OP s concern is whether the suspension post in the saddle helps with back pain. I would think it does . The tyres are highly pressurised, and would otherwise transmit a lot of vibration up the back. There are tarmaced roads in my locality which a very bumpy still, due to the size of the gravel used and I would hate to travel these without suspension. I can feel this vibration even through the suspension post. In fact my 50 year old Moulton does a better job of removing these vibrations . Some commentators suggest that the step through frame is slightly less rigid than a cross bar frame and will therefore damp out more vibrations. If vibration is a real medical issue then probably a bike with a hinged backwheel suspension (e.g mountain bike type )might be necessary.
The riding stance on the Motus is upright with the human back in line with the seat post . I find it comfortable.
In any event the size and the quality of the fittings of the bike will draw attention and there is no disguising it's electric role. I n the step thorough version, The battery pack fits under the carrier and it could carry panniers.
I would not be confident of its role as a off road bike and hope never to put this to the test. The riders stance is very high, the wheels are narrow and it does not have the MTB type rear suspension. On the other hand it would is a great road tourer or commuter bike