Do you think https://openrepos.net/content/osetr/storeman-installer is the wrong place to mention the setting or even briefly describe the install process so newbies like myself don’t have to search the web for hints? Something like this would have given me all the answers I was looking for and this whole subthread would not exist:
To install Storeman Installer, enable “System → Security → Untrusted software” in the phone settings, visit this page using the phone’s browser, and tap the harbour-storeman-installer-$version.jolla_.noarch.rpm link further down the page.
I get it, I’m the first one to complain so what’s the issue, right? 
Could it be that I’m the first isolated newcomer in a long time, and all other installs are done by longtime users? I mean, Sailfish is not exactly a mainstream phone OS, I wouldn’t be surprised if 97+% of its users used it since Jolla 1 or earlier and the rest were their spouses, kids, and converted colleagues.
To add a bit more light from another angle: I do have some GNU/Linux knowledge; I wrote extensive tooling around zypper while I worked in SUSE and it still didn’t occur to me that the browser would default to installing the downloaded rpm. As I hinted earlier, I expected the installer to be in the official store, and when that turned out to be false and there was no info and the page includes run “devel-su pkcon refresh” in the terminal app, it seemed I was expected to type zypper ar https://repo.sailfishos.org/obs/home:/mentaljam/4.2.0.19_armv7hl/home:mentaljam.repo into the phone and that’s clearly ridiculous… Another reason I didn’t expect the rpm link to be the install procedure is that the OpenRepos webui I get on my XA2 with Sailfish 4.2 is clearly not meant for phones.
Long story short: it is not obvious (to everyone) and the fix is a short sentence, please add the blockquote above the the Storeman Install page, ok?
Anyway, thanks for the software, and your attitude. When I was researching what to install the contrast in approaches to users between the LineageOS XA2 port maintainer in XDA forums and your writing here and elsewhere drove me to Sailfish. They owe you a chunk of that 50EUR. 