hey! yup, the components were developed with the idea of multiple panels in a single plug-in. indeed rendering a uxp-panel from React doesn’t work* because, by the time the component is mounted, the entry points have already been set up and UXP has already grabbed its panel roots. the way it solves this is by creating these uxp-panel roots at start-up and later portaling into them. so essentially what you were trying to do is already built into the component, but i wonder if you ran into any issues that led you to do it in the first place?
*though i do find it weird that nesting a uxp-panel inside of the pre-existing one doesn’t work, but i guess that’s just how UXP handles that custom element. given that it’s already handled by the Panel component, you should just avoid uxp-panel.