I think they also have a strong moat @PhilO because hospitals in particular and radiology specifically in PME's case are slow to embrace change - they will generally stick with what they've got already unless (a) the new alternative is streets ahead and offers real tangible benefits which might include significant time savings, or (b) everyone else in the industry is changing and they don't want to be left behind.
So ProMedicus was started and developed by radiologists who know their stuff and they are clearly the best at what they do in terms of radiology software - storing, sending, receiving and analysing images electronically, and it took a while to break through but once they started making sales into major hospitals there was a snowball effect. The moat is the industry being loath to change, and therefore a potential competitor has to have an offering that is not just a little bit better than what ProMedicus do, it has to be HEAPS better before it would even get considered.
And you've hit the nail on the head in terms of PME's annual fees being such a small fraction of their clients' overall annual opex, so PME can push through price increases without much risk of causing churn. In simple terms, they can charge what they like because once their software is in use, it is very unlikely to ever get replaced.
So, yeah, you're honing in on the competitive advantages that PME have, and it's all about the quality of what they offer, the massive advantages their software has over what was in use in that industry previously, and their clients being price-insensitive (not particularly worried about the cost) as well as loath to change to an alternative provider.
The reasons why they are loath to change is really about risk management - nobody wants to be the first to change and risk having their systems fail or not work as designed and that possible system failure potentially cause legal liability issues if patients' outcomes are adversely affected - so there has to either be a massive incentive (which is usally better outcomes and time saving) or else they will change because they see the market leaders changing and they don't want to be left behind in terms of using state-of-the-art tech. It's always way less scary to follow after others when they have proven it's safe to go there.
Anyway, that's my high-level view of why they can charge so much in terms of massive gross margins, even compared to other SAAS providers, and SAAS itself tends to have high margins anyway because of the basically-fixed cost base where the vast majority of extra sales go straight to the bottom line with providers who are already profitable and have their business structure sorted already. ProMedicus have the advantages of being a SAAS provider, AND servicing an industry where churn is really not going to be an issue as long as they maintain their market leadership. I think all the stars have aligned in PME's case.
Disclosure: I haven't held PME IRL for years having sold out at well under $100 because they looked too expensive (overbought) to me at the time (and ever since), but that was clearly a mistake on my part, eh!! I do hold a small position here in my SM portfolio, but I've trimmed that position all the way up and now only hold 7 PME shares here. I don't tend to dwell on "What if..." scenarios in relation to my past investment (/divestment) decisions, however, in the case of PME... ...let's just say it's difficult not to occasionally do a back of the envelope calc of what those shares would be worth now if I'd kept them all.