If I edit the config (e.g. enableRarityParticles, enableCurseFormatting or hoveredItemScale) it has no effect
I tried restarting the game (multiple times)

The changed values remain in the config file but the mod doesn't use them (i.e. it uses its default values)

try the latest octolib and immersiveui versions, not that I changed anything, but I just checked and it's all working, you can also reload configs on the fly using /octolib config reload immersiveui