Ideally FF updates to the latest Spine Runtimes. FF does a number of optimizations in their integration that are neat, but likely unnecessary. It would be simpler and easier to update to newer Spine Runtimes in the future if they simplified the integration, basically just rendering the vertices from the Spine Runtimes.
You can try exporting to JSON with 3.5, then importing into an older version of Spine. 2.1.27 is over 2 years old, so this may not work well. You can try the JsonRollback tool, explained here:
Spine editor and runtime version management
You may also be able to hand edit the JSON. The goal is to match the 2.1.27 JSON format so you can import it into 2.1.27. If you want to know how something looks in the 2.1.27 format, you can create it in 2.1.27 and export to JSON. If you used any newer features in 3.5: shear, paths, keying anything that didn't exist in 2.1.27, etc then you will need to either delete those sections from the JSON, or stop using those features before exporting from 3.5.
