You could add Ombi to your software stack. It supports multiple push notification providers (WhatsApp is included I believe). Bonus is it will let your users add requests which can either be admin approved, or automatically added to your respective *arr downloader.
Jellyfin: The Free Software Media System
Current stable release: 10.10.3
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Sorry not an answer about the notification issue.. Wanted to ask how you managed to get work without issues for every client? What format/codec is you media? Are you transcoding everything? I got some issues with smart TVs.
I've had an issue with an LG smart TV (not really an LG, its a re-brand). The web browser was horrifically slow and it flat out refused to play anything. It has very limited processing resources. I was planning on side loading the jellyfin client for LG TV's but that was all dropped in favour of a chromecast. Instantly solved the problem (and they're quite cheap on social media marketplaces)
Other than that, I've never had an issue with any client.
My media is a mix of pretty much everything available. 264, 265, avi etc etc. I occasionally have an issue with 265 media, stuttering. Easily solved by deleting and redownloading a 264 version. Transcoding (or not, dependant on client) is all handled automatically by Jellyfin.
Hope that helps?