19mm decking is adequate and I can't see any significant advantages of stepping up to 22mm.
The larger the gauge of decking screw the more holding force it has. The compromise is that the heads also get larger and more noticeable. 12 gauge stainless steel decking screws would be more than adequate for your deck. 50mm would be fine but if you did decide to use 22mm decking I would suggest 65mm instead.