I have a similar problem - a large, old, galvanised iron cold water
tank which should be replaced some time - and had been thinking angle
grinder until I saw mention of the 'power nibbler' idea.


Power nibblers are pretty slow and might not cope with the thickness.
You can get air powered shears which are faster, but I'm not sure if
mine would manage a tank either(certainly not the angle iron bracing at
corners).

I'd go for AG, but you should have an assistant watching out, with a
garden pressure spray just containing water. Use a mist to dampen
anything flammable that is getting a serious spray of sparks. The
problem with an AG is that you need to concentrate on where you are
cutting, so can be spraying sparks on a bad place without realising, and
smouldering stuff can catch light out of sight.