Try Impro-Visor you can paste the chords with commas in the Textual Entry box at the top, for more chords on a bar use " / " for beat spacing between chords ,C / G /, your slash chords will be ,C/G, with no space.
You can also add in Utilities > Leadsheet Textual Editor
Any chord name it don't recognize you can go to Utilities > Voicing Editor.
You can then export to musicxml that is dragged into Reaper then "MusicXML Chords" button.
It can also be imported into Biab File > Open Special > Open MusicXML File...
and import the Impor-Visor or Scaler midi also.
ReaTrak musicxml chords notation events to regions.lua