Sometimes I think it's best to discuss this with your chiropractor (if you have one) then they can check your horse over to make sure there is nothing wrong phyically (just in case) then advise you which route to go down and what you can use to help, I think you have to be careful because in some cases you can cause more damage using certain gadgets, there could be a reason why your horse can't develop the correct muscle/topline just from riding/basic lunge work alone.
Anyuthing that gets the horse working from behind and engaging their hind legs using the abdominal muscles will help
A pessoa is a favourite, as you can adjust the way the horse is working. Using trotting poles and raised trotting poles can help, slow hill work in a correct outline, and just consistently working in a correct outline will all build a topline.
I find different training aids work best for different horses - it may be trial and error to find which is most effective for your horse.
A chambon is my personal favourite - it doesn't restrict their head carriage too much, allowing them to stretch all the way down to the floor if they want, and encourages a long, low outline which really helps them loosen up through their neck and their back.
My mare has been lunged regularly in a chambon for the last few years and is now at the stage where even with no gadgets on at all, she carries herself in a long, relaxed outline and works through her back rather than being tight and hollow which she always used to be.