At some point I was in a similar situation. I was running LE and Tvheadend server on a J4205 and considered it not being the best solution for Kodi. I turned the J4205 into a NAS (openmediavault) including a Tvheadend server and turned a raspberry 3 into my new Kodi client. Thinking of moving to a RPi4 once the drivers get more mature.
Turning your G4400 into a NAS and getting something lightweight for Kodi makes total sense imo. No need for for the J5005 except you want to upgrade the G4400 at some point. The J5005 is a great NAS platform.