I dont think weight trainings is the optimal way to increase efficiency or cadence for road riding however higher rep exercises performed on a single leg would be more optimal than bench pressing or doing curls
Cross training is good but the best way to get good at something is to do that exact thing (IMO). I believe track sprinters increase their strength in the gym to help them push bigger gears but they still spin their legs very fast by doing more relevant training like downhill sprints and following the motorbike.
Plyometrics are also good but dont do too much high impact stuff all at once unless you wanna **** your knees haha