It is a good job that isn't the process in Scotland then.
As I explained earlier, the Sentencing Council for Scotland cannot approve its own sentencing guidelines. Its guidelines must be scrutinised by the High Court, which can approve (in whole or in part), reject (in whole or in part) or amend the proposals. The Sheriff Appeal Court can also require the Sentencing Council to review the guidelines and, if they do, the Sentencing Council must review the guidelines in the light of the Appeal Court's reasons.
In England & Wales, on the other hand, the Sentencing Council does indeed approve its own guidelines without scrutiny. For adult sentencing, the purposes are set by legislation. For youth sentencing, the Sentencing Council sets its own purposes. So what you have actually described is the process in England & Wales for setting sentencing guidelines for young offenders under 18, not the process for Scotland.
Also, judges have greater freedom to ignore the sentencing guidelines in Scotland. In England & Wales, judges are required to follow the sentencing guidelines unless they are satisfied that it would be contrary to the interests of justice. In Scotland, judges can step outside the guidelines for any reason provided they state their reasons.
And no, there is nothing dodgy legally here. If this is the process set out by legislation (which it is), it cannot be dodgy legally unless it breaches the EHRC (which it doesn't). You may think it is inappropriate, but it is clearly perfectly legal.