I'm in the same boat. Injured back for around a year that refuses to heal.I'm already using the lowest pedal assists to counter the weight of my bike. An extra few lbs will not make a difference. On a long enough scale, everything breaks down and needs replacing. No bike is maintenance free. I've heard all the arguments for and against full suspension. Those who argue for, aren't just arguing for comfort. My spine is riddled with arthritis, I have shot disks, one replaced and one fusion with more to go. I walked with a cane for two years before my laminectomy. Also have two shoulder surgeries (done in my mid 40's), same ones my father had in his mid 70's. I'm a survivor of chronic pain in the most literal of sense. I shouldn't even be alive. For me and others those bumps and dings no matter how small can truly cause harm and it all adds up. There are pros and cons to everything. For me and others like, the downsides of good, well tuned, full suspension are far outweighed by the benefits. I'm getting a suspension seat post, because that's what I can afford. My bike is a fair weather only, upright, commuter, not a gravel or mountain bike. Full suspension makes sense.
Same story if I were going to buy a moped or motorcycle, I'm sorry, but I wouldn't even consider either if my only option was hard tail with a spring saddle. To each their own, but I hope some of this helps you understand why we want full suspension. I'm not attacking those who don't care for it, for silly reasons or, for legitimate ones like you expressed.
Are suspension seat posts any good for people with lower back problems?