Ah, the Great Awakening. I had one at ~62 and another at 70, when it has become obvious that what used to work is not longer enough. I call it trying to train smarter, since we mostly can't train harder.

Firstly, the very best back exercise in the world is riding your bike hard. That said, as we age, that's not enough. Our recovery systems can't take that much climbing and all out 45" hill sprints anymore. The weight room can load up our muscles with a much smaller training stress load.

So:
Incorporating Weight Lifting into Training
Introduction to strength training for the endurance athlete
https://trainright.com/strength-trai...k-for-cycling/

Lifting heavy is the best, but without a weight room or money to spend on short-term equipment, cheap dumbbells from Amazon work OK. I bought 2 sets: https://amazon.com/gp/product/B071WSFSGC/
https://www.youtube.com/results?sear...with+dumbbells

This is the best lower back work for cyclists, back, hams, and glutes all at the same time:
https://www.youtube.com/results?sear...with+dumbbells
Be careful at the bottom with these. Come to a complete stop, no jerking.