Diagnosis
Numbness of the big toe is suggestive of injury to the spinal nerve of the 5th lumbar nerve. that nerve runs out between the fourth and fifth vertebrae. Even though there is no report on the MRI of nerve impingement, clinically the presentation contradicts the MRI. a normal MRI is not very accurate at picking up nerve impingement although it is good at picking up lots of other spinal disorders. MRI Myelography may be better - a more specialised MRI but I don't know how accurate it is.
Of course there could be other sources of impingement of the nerve but given you have accompanying back pain nerve root impingement would be the most likely site.
Treatment options
Did your physio try manipulative techniques? I would think going to a good manipulative physiotherapist may be worth a try. Such techniques as rotation mobilisations, intermittent traction might be selected by the therapist, based on the findings from the examination. sometimes a system of exercises based on the Mckenzie method can help. There are also other exercise programmes other than core stability exs that can be helpful
Do these techniques work? Unfortunately evidence for such techniques is limited. There is strong evidence that traction doesn't help sciatica. Other manipulative techniques have varying results. Exercise therapy has been shown to be beneficial in the long run over no exercise.
One good thing about a manual therapy approach is if it is going to help you get results quickly. So if it isn't going to work you should know after a few sessions.
With steroid injections - many people prefer to avoid this. Why are you not keen on this option? I am asking as you may have had a bad experience or no someone who has. In terms of the evidence there are a number of good studies but no conclusive results - some studies support the use of injections and some do not.