I (along with every other response here) can only truly talk to my own experience and diagnosis, and anything I mention has to be taken in the context that each individual case is different, and should always be confirmed and guided by a medical professional.
Based on your description, it sounds like our symptoms are/were very similar. I suffered from a fairly significant amount of sciatic pain that increased in severity over a few months. An MRI helped confirm a large focal disc extrusion at L5/S1. It's been a long road back to full fitness, but after 2 1/2 years I'm now back training pain-free.
Things that helped:
- Naproxen for the pain/inflammation caused by the disc on the sciatic nerve
- Lots of mobility work in the early days, progressing to core stability work then now spending a lot of time on strength and conditioning in the gym. Unfortunately, once injured there is always going to be a higher risk of recurrence with disc problems, so my physio has guided me to do as much as possible to strengthen the muscles surrounding the problem area
- A high-quality, physiotherapy-led bike fit. We eventually narrowed the principal cause of the initial injury down to the wrong size of bike frame, which was crunching me up too much in the aerobars and putting far too much pressure on my lower back. I'm now riding a size bigger frame, with approx 55mm more reach. Interestingly, the fit also lowered my stack by about 5mm
- At the peak of the injury, I kept mobile. Zero activity that would cause stress in the injured area, but lots of walks and the odd (easy) swim
- I've revisited my work setup (I work mostly from home). No more slouching on the sofa and making sure I have proper back support when sitting at a desk
Things that didn't help:
- My GP tried a few different neural painkillers. Started with amitriptyline, then gabapentin & eventually pregabalin. None really had a material impact
- Trying to ignore the problem & push too hard. I had a couple of "false starts" where I pushed too hard, too early and ended up regressing symptom-wise
- Wanting to go back for multiple MRIs. I'm based in the UK so the situation here may be different, but the guidance I had through the NHS was that they would treat based on symptoms rather than scans. The initial scan located and diagnosed the issue, but going back for more wouldn't help with my recovery
For context, a cortisone injection was never an option as I have an acute phobia of any needle. However, based on my discussion with a pain consultant, the impact was only ever likely to be temporary and wouldn't address the underlying problem.
Sorry not to be able to present the "magic bullet", but if my experience has taught me one thing it's that the single most important element of recovery is time. It's frustrating as hell, but I had to keep telling myself that I was going to come back stronger as a result of all the groundwork I was laying through S&C and gym work. Time will tell if that's the case, but up until COVID put a halt on any racing (and pre-race hard sessions), I was definitely in the best form of my life.