Since you are low on available space, I would first clean out the bits of Windows which could move to D:
Put not only Users and their data (including My Pictures and My Music) on the second physical drive, but also other Windows files as well. The swap file and page file (C:\swapfile.sys and C:\pagefile.sys), %TEMP%, and %TMP%, all can live on the HDD, instead of the SSD.
However, if you use hibernation, don't try to move the hiberfil.sys out of %SYSTEMDRIVE% , i.e., C:.
When you have made space, then install 19.10 anew on the SSD, reboot into Ubuntu on the SSD, and set /home to where it is now on the HDD.
user68186 has wisely suggested moving /home files to the SSD then creating sym links to the folders with most data (Documents, Music, Pictures, videos etc.) in a "Storage" partition which is NTFS formatted, so both Ubuntu and Windows can read and write to it. That, however, was in the context of having 60 GB free. If you can't shuffle your files around to get to at least 30 GB free space, you may not be able to practically get /home onto your SSD; and please avoid putting /home directly into an NTFS partition.
/home
in the SSD along with the Ubuntu system partition/
./home
to the HDD and use sym links, as I explain in this answer