I post content. You view content. I moderate content. You no like, you no pay
Forcing subtitle burn in would require every item to be transcoded.
In Jellyfin you need to go to User Settings > Subtitles > Burn > All.
And probably change subtitle mode too to always have them on.
Then in Jellyfin add-on for Kodi you need to make sure you go via the jellyfin server (add-on mode I think?) not native mode.
No need for nonsense.