Thevenin Equivalent Resistance of complex circuit
This one is a good exercise for the extra-element theorem or EET which is part of the fast analytical circuits techniques or FACTs. The cool thing with this approach is that you can break a quite complex arrangement such as yours in a succession of small individual sketches all independent from each other. Should you make a mistake, it is easy to solve the guilty drawing and fix the whole thing

With the EET, you identify an element in the circuit which bothers you when determining the resistance \$R_{th}\$ (or any other transfer function - yes, a resistance or an impedance is a transfer function) in your network. Here, \$R_4\$ is causing problems and I will set it to infinity (remove it) for the first approach. In this mode, I will calculate the reference resistance or \$R_{ref}\$ when \$R_4\$ is gone. Then, I will determine the resistance offered by \$R_4\$ terminals when the excitation \$I_T\$ is 0 A or the response \$V_T\$ is 0 V. When you want to determine a resistance or an impedance across some connections, you connect a test current source \$I_T\$ (the stimulus) which produces a response \$V_T\$ across the terminals. The resistance is simply \$R=\frac{V_T}{I_T}\$.

I have arranged all the steps in the below drawings with a small intermediate EET in between. Nothing insurmountable, I did not write a single line of algebra, just did inspect the schematics:



Then, a quick dc point analysis with SPICE tells you if what you computed with Mathcad is ok or not:



The results are gathered here and confirm the dc points analysis. The expression describing \$R_{th}\$ is quite ugly : )