Welcome to TVPaint Community forums !

improve the flexibility of adding guides by...:
right click one of the existing guides in the stack for a pull-down with options like:
-select
-duplicate
-delete
-save
-export/import
(-edit (you could keep the global 'edit guides' button on top too))

when i'm putting down a circle fx while another guide (for instance grid) is active.
the grid 'catches' the outer diameter. this is nice and can be usefull but i would also like to be able to make the circle center 'lock' on the grid.
so i'd like to see a toggle for this.

Any reaction?

for example, for my current job, i 'd like to be able to duplicate guides in the stack...

We are planning on improving the guidelines. Some of the options you mentioned are already in our TO-DO list

cool!