You need to check the accuracy of the gauge. If you haven't adjusted anything then the pressure of the boiler should be the same as before.
You could temporarily hook up another (known working) pressure gauge to the steam wand via a tube and hose clamps and see what the true pressure of the boiler is.
The gauge can not be taken apart but if it is scaled up inside you could try descaling it. This worked for me (gauge was tardy in response/sticky). I hooked up a meter or so of hose to the gauge and filled with descale solution and left it for a while, gauge on bottom of hose, hose vertical. Then flushed and repeated. This fixed it.
Te safety valve should not be hissing. Either you are approaching its set value or it's not sealing properly. Checking the true boiler pressure will narrow down the issue.