My solution is a longshot but may apply to you.
I had a peripheral, namely divmmc sd card reader plugged into my +3 and using the exact same cable you have. Had a very dark picture, despite being connected directly to the ossc.
I used some contact cleaner on the spectrum’s interface slot and plugged it all back it. seems some gunk had been causing massive picture interference. So my only suggestion is pull your speccy apart and give it all a good clean, it may work!