There are a few things to keep in mind when using IL2CPP.
You can create a link.xml file and place it in the assets folder to avoid any code stripping issues with the framework. This prevents all code from being stripped. It's possible there is a more specific version that would still work.
<linker>
<assembly fullname="EcsRx">
<type fullname="EcsRx.*" preserve="all"/>
</assembly>
</linker>
If you create an IReactToDataSystem for a value type, it will get stripped and cause issues. You can work around this by wrapping the value type in a class, or just don't make IReactToDataSystems for value types.