No one can really answer your questions. There are too many unknowns. Nitrates up to a certain level can inhibit
corrosion in
steel but above a certain level can increase
corrosion of
steel. In addition to SRB's (sulphur reducing bacteria) there are also nitrogen reducing bacteria. Not likely a major concern in your case, but it's hard to give a definitive answer.
We also don't know what your
holding tank is made from other than 8 mm steel, no info on specific alloy, or potential coating systems applied.
From your description so far, a best guess is your smell is coming through your connecting hoses given their age. But it could be a vent problem. If the tank is not leaking, it's not coming from the tank.
An expression that comes to mind "if it ain't broke, don't fix it"...
dj