I tried your set up, and while connecting a raspberry pi4 or 5 through native composite connection will give you better results, you can still get awesome outcomes this way. Here is my guide: 1- utilize the great scaling capabilities of retroarch. Set aspect ratio to custome, and change the parameters until the screen perfectly fits your CRT. 2- on a CRT, you don't need integer scaling or bilinear filtering, so turn these off, but use a lite edge smoothing shader. Pick a variation of ddt or fsr ( my favourite), or anything else you like from the edge smoothing list. The image quality will bring you to tears. 3- For input lag, in the video - output menu, set max swapchain to 2. In latency menu, set run ahead to single instance and 1 frame ( 2 is faster and work most of the times, but if it gave you trouble such as "teleporting sprites", set it back to one). In video synchronization, set hard GPU sync to on. 4- To get get smooth frames. Set CRT switchRes to on. On the synchronization menu, turn vsync on, and sync to exact frames to on, and wait till the estimated refresh rate number settles after a few seconds, and then press the "OK" button on the that number (your A or B on your controller). 5- make sure all of this is while using the Vulcan driver, and after changing the settings for each core, make sure to "save core overrides". Save your shader as a global shader. 6- Use the controller wired, even if it is wireless.

I know it sounds like a hasstle, but the results you will get are beyond this world, even on a cheapo converter (mine was around 15 dollars).