Binaryeye supports both decoding and encoding via Share menu. So you can share an image from any app to decode(can also be a photo that has a qr code somewhere). It also supports sharing a text or link to generate one. It also has searchable history support.
Of course Github is just an example but you can pretty much regex any URL and further filter out anything in order to get the apk link with it. So depending on your level of privacy requirement and trusted sources, you can skip all the centralized ones and build your own list of sources.
Not exactly answering your question but you can use the app Obtainium to fetch the apk URL from a website/github repo and many other sources to install directly. It also supports fdroid repos and many other sources out of the box. Kinda half way what you mentioned in your first paragraph.
Seemed so nice until I tried to add my very first personal server that has Oracle Linux distro on it and it paywalled me immediately. So if you want it for personal use but you use the wrong(!) distro on your server, tough luck! You gotta pay for it unless you replace your server with something like Debian I guess. That was the end of it for me. As a constructive feedback: it would be nice to see a list of which distro/server os variants are not paywalled, or which ones are paywalled. For now Asbru will do it for me.
Edit: turns out it’s written out on the pricing page in detail. See the comment below.
That’s interesting, I’m using it since android 4 days, don’t remember a single crash. If that’s the only case it crashes, I would open an issue and use it in the meanwhile. Check the permissions maybe?