Use a tiny amount of silicone lube on the zci rotary oring to free up movement
Try the ec gearbox, nothing to lose. G&p older gearboxes aren't an improvement over ec gearboxes generally but over the years things have changed. My old g&p from 12 years ago was trash for instance.
Might run into some fitment issues with the ec though. Personally id just transplant the titan or buy a new perun for the g&p, open it up, check the parts and see what needs to be improved.