What joint should I use for this 45-degree piece? It provides lateral support to a table leg, and there are two (one on each side of the upright piece in the picture). The bottom is a 3-way mortise and tenon. I'm reasonable comfortable making those now, but cutting the tenons on the 45 could be a bit challenging, especially since I'd have to do 16 of them for the whole project.
You can just use butt joints here. Since this is part of an overall structure the joints will be strong enough in service.
You can reinforce them if you prefer*, but it shouldn't be needed if the pieces mate well, enough glue is applied and you clamp well during your glue-up.
*With nails or screws or dowels or biscuits or splines or Dominos or floating tenons.