Regarding rtContextLaunchProgressive2D

Reading some threads about the progressive launch mechanism, it’s been mentioned that’s it’s not recommended to use it on a local system. What would be a recommended way to make a local progressive render work with, say, a GUI, then? The progressive launch is non-blocking, which seems ideal for an application like this.

The progressive API is effectively a non-feature in OptiX 6.0.0 because the support for remote devices has been removed and that API was only provided on local systems for debugging before.

Simply implement any necessary progressive accumulation inside your own ray generation program.
That is going to be faster and more flexible on a local GPU system.

If you only have one GPU for ray tracing and rendering the GUI, there will not be a benefit for the interactivity because the GPU is going to be fully loaded by the ray tracing work in either case.

Please have a look into the OptiX Introduction video and example programs which are implementing a fast and elegant progressive path tracer.
https://devtalk.nvidia.com/default/topic/998546/optix/optix-advanced-samples-on-github/
(The code on github is not yet adapted to the OptiX 6.0.0 SDK library names. See other threads in this forum.)

Note that for performance it’s also recommended to put variables which are changed on every launch into a small input buffer and update that instead of setting declared variable values.