1. You should try a more recent build which includes more recent duckstation builds for psx games or just try out beetle-psx, pcsx_rearmed
2. The J5005 isn't exactly a highend system & from my personally experience with my J3455-ITX some simpler GC/Wii games will run but don't have high expectations especially if it's a passiv cooled system. You should also focus on dolphin standalone which ES starts, the dolphin libretro core is pretty dated.
3. You should use PulseAudio as audioserver, Kodi only relies on ALSA for passthrough usage. If sound is stuttering or else it's most likely because your system does not emulate the games fast enough which leads to audio quirks.
I switched to to beetle-psx and it runs a lot better. After switching to the Vulkan driver in Retroarch, Gamecube runs very good for the most games as well. Very happy with that! When I start the games in ES, the gamepad doesn't work anymore - will investigate on this.
Your are 100% right - the J5005 is not a beast and initially I did not intend to game on it.... But the Kodi hybrid builds seen here just made me want it
Regarding Pulsausio: I would be glad, if you could give me a hint on how to make the switch.
I've read through this but still stuck on the configuration:
PulseAudio - Official Kodi Wiki
Thank you!!