Is taking a postdoc for training purposes wise as an industry scientist? I'm considering taking a postdoc after 5 years as an analytical chemist in the pharma industry (small molecule). This may seem backwards and counter-productive due to the massive pay cut but here's my reasoning:

I am extremely unhappy in my current role; I've been pigeon-holed into developing methods for a rather
niche technique that isn't versatile and incredibly uninteresting.
I've also been pushed into pure project management and doing little
to no science.

When searching and interviewing for roles I want, it has been
highlighted that, while I have industry experience, I am not
experienced enough in mass spectrometry or large molecule work, which
I do not have the opportunity to train at my current job. I've been
searching for 6 months now with several interviews that all came back
with the same feedback about my lack of mass spectrometry or biomolecule experience.

A postdoc position is likely available to me through my personal
network and it would be an intensive way to train in mass spectrometry and large
molecule analysis (as well as some data science and bioinformatics). There is a grad student who currently handles most of the technical aspects of the instrumentation and sample prep (he'll be my immediate mentor) while the PI is more on the bioinformatics and publication side of things. I will be asked to help conduct analysis, maintain the instruments, and write papers (the PI has mountains of data but no time or desire to write the papers).

My logic is that, with 2 years of focused mass spectrometry training and 5 years in
industry, I would be a more attractive candidate and have more
options in terms of where I might land a job (different industries,
possible academic facilities staff, or national labs staff). I would also have materials for presentation that highlight my competency whereas right now, I only have my PhD work (in a different technique) and a generic industry presentation (cannot use company data so I can only give lectures on general topics in pharma).

My long term goal is to shift my expertise to mass spectrometry and either join a national lab, core facility, or build up an analytical department at a startup. Mass spectrometry is incredibly versatile and is a critical hard skill in pharma, biotech, food science, environmental, and other key industries.


Am I thinking about this correctly? Or am I making a mistake be treating the postdoc as a training opportunity and stepping stone?
Sorry, this doesn't seem to be a good idea, especially considering some of the more subtle points you've shared. In general taking post-docs after 5-6 years need not be a bad thing, but people would generally shoot for a prestigious fellowship, not something through personal contacts.
I also want to recommend that you take this just because of how unhappy the current role makes you, but considering your situation I can't. You're in that tricky (not uncommon) position where your PhD, current role and future aspirations are in different fields. As such, you should be very wary about letting go of what you have for something that you potentially may get. There is every chance that the grass appears greener on the mass spectrometry side, and after being trained on it (by a grad student!), you may find that there are difficulties and unpleasant challenges that you aren't currently aware of.
You seem to move towards mass spectrometry in the long term, and I get the uneasy feeling that you're jumping onto the bandwagon without enough thought. Primarily because if you really want to make a big switch, getting a nameless (i.e. without a fellowship) postdoc and being trained by a grad student (and neither earning certification nor being under the wing of an experienced academic) doesn't sound wise.
There's every possibility that I'm assuming too much, so please use your discretion and don't be hasty.