SF86P suitability concerns-- should I talk to manager or adjudication department?

I am concerned about potentially being found unsuitable. I wrote elsewhere about concerns related to unfiled taxes. I feel like that alone could sink my prospects. But I also have a history of cannabis use and a recent episode of mushroom use experimentation. I personally believe these events are in the past and I have moved on. I intend to give full, honest answers and hope that they demonstrate that repetition is unlikely. But obviously I am my own most sympathetic advocate. My research here and elsewhere leads to me to conclude that I probably have a low chance of passing the suitability test.

In a situation like mine, where a person feels they may be unlikely to pass, what should they do? I’m supposed to start the job soon and begin receiving training. It feels rather silly and pointless to start if I think there is a high probably that after several weeks I will learn I can no longer work the position. I regret that I did not understand the scope, standards, or timing of the background check before now. Does anyone know if it is possible or advisable to speak to either my manager or someone in the adjudicatory department of this agency about how this is likely to unfold and whether I should proceed or withdraw?

I want to give lots of thanks to the people who manage this forum and the people who post answers. This forum has been a helpful resource for me in pondering various potential issues.