Gt3071r (56 trim) does have a tendancy to surge under 4500rpm on standardish YB 2.0 engines when running over 1.8 bar.

Sounds to me like you allready know the answer:

Changing housing to 0.82 will increase spool threshold to 4000rpm and reduce turbine back pressure. This alone may cure your surge.

Changing to anti surge housing will reduce compressor efficiency. However you should have chosen a different compressor wheel in this case as why have the larger 56 trim wheel, only to reduce its efficency with a ported anti surge inlet!

Cap the boost via your ecu mapping until it can take full boost.

All your other options require the engine coming apart Im affraid to build something a little more suited to the turbo! (Should have chosen a suitable turbo for a standard engine!!)