As a whole the GIS world is...surprisingly shallow. There are some technical niches for sure, but compared to what I've seen in other domains of engineering, one can get incredibly fast to a point where one can do 80% of all GIS work. Really, a basic GIS course combined with a modicum of data-wrangling chops and Google skills can get you very far. To speak from personal experience; I had 2 mandatory GIS courses at uni, took one Python+GIS elective, and learned enough on the job the past 4 years (all of ArcGIS Online + PostGIS + ArcPy) that I can prolly apply for most senior GIS jobs out there. A lot of GIS work is just about getting the right input into the right GIS tool(s) and ✨presenting✨the result. I know people who have done nothing more than "load data into GIS, apply pre-made tools, visualize" for decades. Which for sure is reflected to a degree in salary.

A shockingly small niche (over here at least) is the people who are good at writing queries and half-decent at GIS. PostGIS legitimately can replace 95% of the individual pre-made tools QGIS and ArcGIS has to offer. You can do much more complex things much faster. My largest project the past year ended up being 2300 lines of PostGIS/SQL code I wrote on my own. The first 30% is just data prep written in code - "make sure I properly join tables A thru G in the data type I want it to be without ever having to touch Field Mappings ever again". The rest is a bunch of clever geo-joins and a bunch of not clever regular joins of tables and features. Nothing special to anyone who already knows how to handle semi-long SQL queries; PostGIS is really just one new column type and a bunch of functions to do stuff with it.

For many in the GIS world, once they start seeing the benefit of PostGIS, they often don't go back. It really is objectively better to get shit done. Which means that a lot of skilled GIS people have made a lot of PostGIS code that only a small subset of GIS people can work with. Which means that if you're the kind of person whose brain can be wrapped around the core concepts of SQL and GIS, it's an easy ticket into advanced GIS work.

For me it took a 2-day (intense) course plus a week of actually working with it under a deadline to go from "I can do most anything I want with Arc" to "no wait actually this PostGIS thing rawks". YMMV - I know in the US, the GIS world is much more imagery heavy, and imagery does not gell well with PG. But I can get so* much more work done these days by doing 95% of it in PG from the QGIS Database Manager that it's an easy recommendation.