MIght think about a modified battery unit, have the battery in a pocket or on a shoulder strap separate from the machine. Take some wiring changes if you are somewhat handy like that. Sure take some overhanging weight off that back. I'm getting close to that situation myself.
doesn't work like that Tom.
The head and the engine need to be balanced so your arms are just directing it to where you want to cut rather than holding it up.
What most people fail to do is adjust the harness properly.
With the harness over your shoulder the head of the trimmer should be about 2" off the ground.
Next you need a proper harness, one with both a shoulder and a waist strap.
I did a little presentation at one of our Mens Sheds a few years back.
Aussie blokes tended to think harnesses were a "sissy" thing but once I fitted a few of them and the old farts found they could trim for a full hour without feeling tired or fatigued there was a long que at the Husqvarna shop buying the big harnesses . Down here the Husky one is a lot better than any other as it has big clips that old fingers can manipulate easily.
The other trick is to start it in a heavy vice and leave it there of a few minutes till it is fully warmed up