When attempting to convert large Minecraft worlds from the PS4 save system to the PS5, any worlds with resource packs not available in the marketplace won’t convert at all.

Additionally, if the world file is large enough—about 400 MB or more—it also won't convert, regardless of whether the resource packs or add-ons are from the marketplace. My main world is around 390 MB raw, plus 10 resource packs and some other extras, totaling roughly 400 MB, so I couldn’t convert or play it. Here’s a video showing me converting a smaller world, as well as my main world.