As to changing the cable I think 16mm2 would be preferred, but if you plan on running at 30A charge most of the time arguably that's what your existing system has been doing and I assume no signs of any heating issues?
If you wanted to regularly charge at the full 50A I'd be much more inclined to run a 16mm2 while you have everything in bits anyway.
If you wanted to regularly charge at the full 50A I'd be much more inclined to run a 16mm2 while you have everything in bits anyway.