I'd suggest the light issue is either due to the thickness of the paint, repairs, or distortions to the wall/plaster. Adding a new skin of panels over the wall will probably solve the issue.
If you weren't keen on filling the edge and blending it into the adjacent wall, you could use external timber moulding as per the installation guide. The reason I like this option is that you're essentially intimidating a V-groove board with these panels. Boards have a thickness that would need to be covered with such a strip. In reality, they don't just seamlessly blend into plaster walls that run perpendicular to them.
It's up to you, though, as you might not like the idea of timber moulding on the corner.
Thanks Mitchell, another good consideration to use the moulding. Once the panel is up I will assess what look / works best.
Welcome to the Bunnings Workshop community @SebastianD. It's wonderful to have you join us and many thanks for your question about this amazing project.
Our knowledgeable members would be delighted to provide advice if you'd like to attempt this project yourself. With a bit of assistance, I'm sure you could look at tackling this yourself. If you did want to employ the services of a tradesperson to complete the work for you, then you might have luck advertising the job on a site such as Airtasker or hipages.
Sing out if you'd like to give this a go yourself. We'd be more than happy to assist you.