Stretching is a given. However for keeping the back strong hitting the gym is also very important as mentioned before

Kettlebell is one type exercise which will help with back problems. Most kettlebell exercises have a moderate to heavy emphasis on the back and kettlebell by nature of the moves increases flexibility and mobility and just opens things up quite nicely.

Body weight exercises are also a blast since they take many muscle groups at a time and usually target the core pretty well. Just a normal L-sit is something every cyclist should be able to do (can't do it yet, it's a tough exercise but hurts so good)

Squats are also good for glutes and lower back but many people get the technique wrong. It's a strangely challenging move especially for cyclists. It requires good hip flexibility in the area where cyclists usually don't have it so learning and stretching to a good squat technique can be really beneficial. Start with bodyweight and when that starts to go well move onto an empty bar or maybe two 12kg kettlebells. Add weight with progress