If the scale which remains is pretty light Id still go down the citric acid path.... dissolving as much as you can in boiling water - and that is very effective - but only when hot.
Dilute hydrochloric is what the professionals use..... a LOT more dangerous but also more effective than the citric. Dilution - dont know what they use.... but start off with a weak solution and see how it goes. HCl is safe to use on copper boilers, pipework, fittings etc. The professionals pull the whole unit down and the boiler and all the bits go into the bath (watched them do it once)..... and at the strength they use it doesnt take very long at all.... and everything comes out as bright pink copper.
Just a quick question..... your heading is "descaling a stainless steel boiler".... so is the boiler SS or copper?