Additional training/degrees (and cost) after postdoc
Finding jobs is hard, and usually one should only restrict the specific type of job OR the location. If you are limited to one location, it would be better for you to broaden the scope of jobs you might consider. If you are sure there is one type of job you want above any other one, then you need to open your geography.
In your case since you are restricted to a location the best is to consider different career options.
For your plan A, your ideal positions, the key will be timing. Those positions might not come up so often so your focus should be to make sure you know the key people at those relevant departments and build a relationship with them. Let them know you are interested in working with them one day. This way you will hear about jobs before others do, and they might even be able to create a position for you. But this is rarely fast.
It is also good to ask them what type of qualifications they had when starting in their current field. That will help you see that (more often than not) people come to those alternative positions through many different paths, so there is no right or wrong.
And for broadening the options that you consider, I love a mental game I read in a book about the "vanishing option". What would you do if your favourite option is off the table? it seems trivial but that time of targeted thinking can unlock many ideas. Those should become your Plan B and C... and you can then learn about those fields and meet that other relevant people in parallel to your efforts on your Plan A.
And you got it: networking is the key (not only to learn about the opportunities when they come up, but to learn about all those other fields!)
I think you've gotten some great advice so far (especially about the importance of networking),but I thought I would provide a slightly different perspective. Regardless of whether you do or do not pursue additional certifications, it's important to keep in mind that everything you include in your resume should paint a picture of who you are as a professional.
As someone who has been a hiring manager in industry in the past, I have seen a lot of entry-level resumes. When I was hiring for entry-level medical writers, new PhD graduates and postdocs who applied were a dime a dozen. Almost everyone applying had a PhD, multiple publications, etc. However, someone who had taken some AMWA certifications or some other writing-related course stood out as someone who was truly interested in a career as a medical writer. Other applicants who were in their 4th+ year of a postdoc and hadn't made any effort to take on any type of writing work (through their university, as a freelance, etc.) or certification seemed less serious about medical writing as a career and more like one of the many people who applied because their grant was running out and they just needed a job. As a hiring manager, I didn't want to hire someone who was going to get their foot in the door in industry, and would then likely resign after 3 or 6 months for another position once they had industry experience.
In contrast, I have seen other people's resumes that are all over the place. They take online course after online course and every certification they can find. These people go in the same bucket as the previous group, as it is not clear from their choices why they are interested and well suited for a particular role.
Certifications can provide valuable expertise, can serve simply to make your resume stand out from others, or both. I recommend talking to people in your field to get a sense of how certifications could help you and how different certifications are perceived within the specific field you are hoping to enter (it differs quite a bit by field, the popularity of certifications, and the weight/acknowledgement that certain certifications carry). Certifications are definitely not required to transition into industry but, in some cases and in some specialties, can help if they mesh well with your overall career goals.
Caroline M. Ritchie, PhD