I spent a few years living in the U.K. In late 90s-early 2000s and I noticed they were a trendy pet back then.
Saw some very neglected ones in filthy tanks and their shells were sort of peeling off the top coating and they were all white underneath.
If they have been living in a tank for so long before you get them do you have to train or rehabilitate them in some way before they can live in the pond? I imagine it's a shock for them to suddenly be living a semi wild life after 10+ years in a tank.
I give them a carrot soak on arrival. This is very good for turtles and all of mine get this treatment on day of collection. I also give them an acclimatising period, especially those kept in warm tanks. To be honest though they adapt very quickly and overall are very hardy and resilient.
The shell problems you described can be a number of things from poor diet lacking the vital vitamins these reptiles need, poor lighting also lacks the vitamin supply for a healthy shell. Lack of calcium can also be a problem that effects the health of the shell and can lead to calcium bone deficiency. It can also be just growth spurts. Turtles scutes are naturally shed as they grow. Dirty conditions can also cause fungal issues with shells.
When you put a turtle outside you quickly see a vast improvement in shell condition. Nothing beats the sun for shell health.
But they are very hardy and adapt very well to life outside.