I would manufacture a prototype now, and see how it performs.
The low pass is ok. I was not sure about the extra capacitance of the pads on the shunt capacitor, but that's ok.
The only thing we could do now: simulate the SMD with measured data/vendor models to include the parasitics, if this data is available.
For the antenna, there will be some influence from tolerances of the FR4 material. I simulated with your value of 4.6, but that value might change depending on the supplier. I think the design is good enough to build a protoype now, with this layout.