30 degrees is very limiting and can imagine the pain I watched many operations on youtube on the shoulder its amazing what they remove. Luckily I caught it early I was unable to carry chimney pots down from roof in daily job for some time as mine was dominant hand.
I remember on achieving the box splits using a manual by a russian author and with anything like this it is very important to do a last thing on a night stretch routine before bed it re-calibrates muscle memory.
There is not fast way out of some of these things it's a long game and if it effect your job you need movement back fast and its difficult you have to adapt.
Dupyutrens contracture runs in my family which I have kept at bay for 20 years by similiar methods where family members have operation. I use full body weight on the hand for 100 reps each day i.e place hand on table if its a finger place other hand over lift your body up off the floor as many times as needed to flatten then onto next finge. and the pain is very very intense for 1st few reps but your hand\fingers gets the idea in the end it needs to be flat. Massage after and hear tissue cracking and you have held off till next day.