I think that your SCR model with two BJT is completely wrong. You can see from the I(R4) vs V(anode) characteristic that I(R4) ranges from positive to negative (and high) values. This can’ t be, because a SCR works alternatively as an open circuit and a diode, depending on the gate voltage and on the voltage applied between the anode and the cathode. I’ve attached a basic SPICE cirduit model of a SCR with two ideal BJT.
Below the anodic current and the current vs anodic voltage characteristic.
It can still be improved by introducing the capacitances beetwen Q1 base and emitter, Q1-Q2 bases, and Q2 base and emitter.