yeah similar to what Illustrator can do or ClipStudio you can also select individual points on the lines you've drawn and change their thickness in the same manner.
guess ill dig up my old bought copy of Illustrator and and use that for now.
I'll grab the trial of VectorStyler and give that a look. thanks for the suggestion Dazmondo77
its a little odd that Designer cant do it as the core functionality for it is already there to a degree with the Graph. The graph however is just a very clunky interface for accurate and detailed control over the line.
was just looking at Designer because Im not a fan of subscription models (Adobe) and I've been using Affinity photo for quite a while on a different machine.