In your position I would give the tank one more concerted try with a hose/jet wash and some detergent and after about 4 thorough rinse outs I would dry the innards out with a hair drier in the filler opening (moving its position several times so you do not blister the paint). If the problem still persists then I would bin the tank.
Such issues can be very frustrating and demoralising - I wish you luck with whatever you decide
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