You'll want to get the save from steams prefix and copy it to heroics prefix, or your default prefix. Sorry, not near a pc so this'll be a bit off
The steam save file should be located somewhere around here:
$HOME/.local/share/Steam/steamapps/compatdata/1245620/pfx/drive_c/users/steamuser/AppData/Roaming/EldenRing
It should be called something like "er0001.sl2"
Find the same location in your heroic prefix, which should be saved here
$HOME/Games/Heroic/Prefixes/EldenRing/pfx/drive_c/users/steamuser/AppData/Roaming/EldenRing/
Locations may be a bit different, sorry for any possible confusion.
If you removed the game from steam already though the save file is gone. This has bitten me in the past personally
Edit: Save file ends in sl2, not sl as I had stated before the edit