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Not GP, but most of the apps I use that works without Google Play Services (specifically, FCM) have this problem too. Vendor-agnostic notification on Android, and as far as I know iOS, is still painful. Ofc Slack and WhatsApp works fine : they use the Google notification system.
I did exactly this for my startup (https://easy.green). I'm very happy with this setup so far, with the code embedding feature for on premise delivery. I had to disable the worker mode though, it caused issues with uncommon features in the ORM.
For lightweight JS interaction, I prefer the Stimulus controller approach that doesn't mess too much with your HTML, and doesn't try to sell you "components"...
I use this with Mozilla SOPS to crypt sensible yaml configuration values stored in a Git repo. Works great, fast, and keys and payloads are short.
I don't totally agree with you on this. Telemetry on open source extension is public and opt-out, but it is neither on the MS VSCode builds or proprietary extensions. Additionally, I just can't believe that Microsoft uses telemetry data just for bug fixing, even without evidences, but that's only my opinion.