As @mankiis and @riban have already suggested, what you really need is to set up your Zynthian with instrument chains that expose the system’s excellent built‑in plugins and let you browse their presets. You may add a few of these chains - e.g. MDA piano, setBfree, and multiple instances of FluidSynth - and they will appear in the mixer view, and can be quickly selected by just touching the corresponding strip. Then you can save them as a snapshot, and that snapshot becomes your quick way to return to your preferred default setup whenever you want.
That’s just the starting point. The possibilities for fine‑tuning your system are extensive, and as you become more familiar with everything Zynthian offers, you’ll be able to shape and refine it to match your workflow. It does take some time to learn and master all the features, but the flexibility is well worth it.
There are ongoing ideas about making this whole process easier for anyone exploring the Keyboard-Expander use case on Zynthian, for example, the discussion here: https://discourse.zynthian.org/t/zynthian-jfk-just-for-keys/. For now, though, this approach is probably still the most practical one.