I tend to stretch after cycling, but not before. I also do 30 minutes of gentle yoga and stretching each morning, partly just to help me wake up and get moving first thing. I'm sure it makes some difference to flexibility, but as with anything I think you reach a 'plateau' and stop improving after a certain point.
If you search for 'yoga for cyclists' on YouTube, there are quite a few videos, some designed for using after a ride, some for general flexibility. I haven't used any of them myself, but it might be a starting point. One advantage of following a 'proper' yoga video is that at you shouldn't be going straight into a strong stretch without building up to it.
One of the few downsides of cycling is that is does stiffen up the body. I have always suffered from poor flexibility, even the Vulcan salute was beyond me. Tried a few yoga classes and disliked them but conceded that yoga is a good practice on the whole. One of the classes in particular was mostly populated by bendy super powered narcissists. Woke bloke who was the leader appeared to be only superficially empathic to middle aged stiff gits like myself. I will finish off this moan creak groan later as I am late for work.
I've found that yoga classes vary by area and time. For years I went to a lunchtime class near work where I was normally the youngest there, and I was in my 40s at the time. Then my job was moved to central London and suddenly I was the oldest person in the classes nearby. I've noticed that a bit of narcissism has crept in over time: my first teacher 20 years ago was adamant that clothing should be loose and cover most of the legs and shoulders, now most people seem to wear minimal slinky gym gear.