If you constrain yourself to mod-ui then you only have LV2 plugins to choose from. I suggest that mod-ui is more limited than Zynthian’s native mechanism. Sure, mod-ui looks pretty but it is prone to crash and had many limitations. But if that is your thing then also is probably the most feature complete looper in the system at the moment. I found out had a few issues that require upstream fixes so I stopped trying to make it work.
If you are willing to step away from mod-ui then we are just about to add full support for SooperLooper to Zynthian. There is a recent thread discussing this. SooperLooper is by far the most feature rich looper available for Zynthian and the current development makes it a first class citizen in the Zynthian ecosystem. I suspect that within the next couple of weeks you will see it available, initially in testing then in stable. The stable release should be accompanied by full documentation.