I'm a kinship carer of my nephew's half siblings, with the long term plan being to get an SGO. So I am by no means an expert, but I can answer based on my experience and what I've been told about the process going forward.
The fact that your stepson hasn't been placed with you temporarily if SS has started the process for an SGO seems very strange to me. It's always preferred for the child to be with family and ultimately an SGO is not usually issued until the child has been placed under a care order first, so the court can see the placement will work before it is made permanent.
Are you sure you are having an SGO assessment and not a Kinship/Connected Person's assessment? Because that seems to make more sense with your stepson not being in your care.
From what you've said, I would say that the court should have set the filing date for your assessment (else it wouldn't have been started) but because it has not yet been submitted then nothing further has happened.
Have there been court hearings not directly about the SGO, as you are not a bio parent and don't have PR you wouldn't be invited to court dates about the child and may not be informed of them (I only knew when court dates were because my cousin was at the hearings due to being bio dad of one of the children involved).
At court hearings relating to your stepson his SW should have told the judge that you are being assessed and that you are their recommended permanent carer, but that is not actually anything to do with the SGO.
If your assessment has started, I would recommend talking it through with your assessing social worker, it is their job to support you through this process. Your stepson's social worker may have some information too. Ultimately, some SW's are better than others (my assessing SW was amazing, but now the Care Order is in place I have a different supervising SW and he is useless. Children's SW is somewhere in between) it's the look of the draw.
Once my assessing SW had filed a positive report my contact with the children was unsupervised, so if your assessment has been completed but has not yet been filed with the court then your contact may remain supervised until the report had been filed but be sure to ask this question, as I was not told that I was unsupervised and allowed to take the children out until two months after the report was filed when I asked about taking them out for a Christmas treat.
Regarding legal representation, I was told right from the start (initially viability assessment, so before the actual assessment) that when the SGO is applied for I will need my own legal representation. There are charities that will support you with this, have you contacted Grandparents Plus? I have been told that they are pretty good.
You could also contact Foster Line for more information about Kinship care and SGO. They may be able to shed some light on the process.
I hope this has been somewhat helpful