I've been DLing enormous files, 100+GB, that can be divided into parts, but can't seem to find a solution to store finished parts offline, because I only have limited capacity on my drive, but TBs available in offline backup.
I have tried to experiment with symbolic links, and hard links, junctions, etc. But if the main file is removed it comes up as "file error" or "file name can't be resolved by the system".
Can you make a feature to make offline storage possible, or easier?

That's a big lift here. It's just a downloader program made for free for users. To do what your asking would require funding source to create such software to do that with and there probably software devs that make such a paid program to do that with. Since FDM is free that would not be affordable for FDM to do that with limited means.