: parameters are 15' 6" by 21-1/2" (0.55 m) and roughly 11-1/2" tall.

That is relatively short. However it is beamier than my present one (0.51 m) that I think is quite stable.

My previous kayak which I paddled for near a decade was tortured ply which meant it didn't have chines to give it any stability. I started with 6 litres of water in wine cask bags as ballast and soon gave up and started thinking "Why did I ever think this was tippy?" Now any kayak feels stable. Meaning it is a great way to become proficient. a great way to learn to brace instinctively and not even notice it.

I recommend fitting hip (and probably thigh) braces, lowering the seat, possibly adding some ballast, up your skills and start reducing the ballast.

Maybe make another more stable kayak for trips where you want to paddle with others in conditions that might make life difficult but use the unstable one as often as possible.

My unstable tortured one was a completely different kayak when loaded for multiday trips. Admittedly it didn't have as much room for gear as I would have liked.

Alex