I also had several fusions and have dealt with similar problems. I do have some trouble in the lumbar area, but my fusions are all in the cervical area. Also cleared to dive at about the same timeframe post-op as you.
As others have said, sidemount can help. You can monkey dive an al80. Get a couple al40's or a couple al63's even. The big benefit comes in not having the tank strapped to you until you're in the water. If you're better on gas than me, you might get away with just using a single lighter al63 or something.
To expand on what @mi000ke said, as long as you're boat diving, a quality dive op can make it work with any backmount setup. They inflate your bc and hand it to you in the water. you don, and dive. When you surface, you doff and they haul it up for you. I did this with dive paradise in Cozumel last year and it was really great. If you ask in advance, I think many ops would be able to accommodate as long as the waves are small. It doesn't require any special equipment on the boat, just willingness on the part of the crew. Ask when you call to book. Show up at the boat early, and discuss it with the crew. If you can get them to do it, this is the best option I've found by a long shot.
I tried using a cart for shore diving but carts can be a bit unwieldy. I switched to a "hand truck". The one I use is a lightweight fold up thing that cost around $25 at ace hardware if I remember right. You just have to carry it back to your vehicle or find a way to chain it up when you're in the water unless you've got someone at the shore to make sure it doesn't get stolen. The hand truck is also useful for bringing my tanks back and forth for fills.
If money is a non issue, you might look into those hoop wrapped tanks. Might be able to shed some weight from your gear with those, but they're massively expensive and have a very short lifespan compared to anything else. Also, they can be hard to find, and may be hard to get VIP/Hydro (don't know). When I saw the pricetag, I stopped looking into these any further.