I made this app! Built it for myself and my partner because I had exactly the same problem — every other medication app seems to be built for taking something on a regular schedule.

https://github.com/cliambrown/PillTime

Not sure if it's very user-friendly, but it works well for me. There also isn't an option for more complex rules (just max X doses every Y hours), so you'll need to set something up that makes sense.

I've got a pending request to have it listed on F-Droid. In the meantime there's an apk on github, and it seems to be available on Google Play and IzzyOnDroid.

Denver Courtroom 209