[SaltyPuglord] needed a solid state relay for a project. We’d have just bought one, but he decided to design his own in LTSpice. Along the way he made the video below, which is pretty informative and a good example of a non-trivial design in LTSpice.
MOSFETs have made designs like this a lot easier, to the extent that it should be as easy as putting a pair of beefy fets in-line with the AC source and load. However, that has a few ramifications that [Salty] covers in the video.
The biggest concern comes in isolating the DC supply from ground. He used a transformer which is tricky to simulate in LTSpice. Beyond that the design of the power supply is quite simple, and as he mentions in the video, you don’t really need this complex of a regulator just to feed the gates of the MOSFETs.