You have done a great job. I would feel pretty good having achieved that level of clean.
I wonder if some of the non-volatile brake cleaner would finish the job for you? If that doesn't touch it, I would consider it good to go myself... [Our tanks
were last mucked out in 2009... I appreciate the level of effort...]
The only advantage to getting it fully cleaned is so you can inspect the welds in that corner...
If you had to leave tomorrow, I would consider that a job well done and fill the tank...