There's a lot of shit I don't appreciate about Android, but setting as many timers as you want is a total no-brainer.
Having multiple timers running is inelegant and sometimes confusing. So instead of giving users any agency in that situation, they just ban the very concept as a whole.
If they feel enough pressure, and they can come up with a satisfactorily simple UX, they will usually eventually fix the hole.
Of course, many apps on the app store offer this feature.
The main annoying part is iOS PWA's kill their service workers when off screen even when explicitly told not to via the `waitUntil` command, so you need to have a server running which handles keeping track of the ongoing timeouts and calls the push notification endpoints accordingly.
Code for that is here, https://github.com/JacksonKearl/push-simple. There's a Dockerfile, currently deployed via fly.io.
Funny that I provide a fully working code sample of how to do what the parent wants, explain the caveats associated with it, and get downvoted because Apple crippled their service workers... ok then, Hackers.