Ok , If you have Android you can find some apps for control data in your internet , Try allowing data only to Noster and close data from other applications. What will happen to you?
Discussion
Your solutions are too simple, I have thought of them all before. The main solution to this problem is that the developers of Amethyst try to reduce its data consumption, for example Primal app for Android has a much lower data consumption.
In my home I use fiber and I don't find any problem with amethyst, I think you can use other clients for nostr like " snort " or " primal " or " plebstr " or " " nostrudelninja "
Try them which one is best for you
I use Coracle most of the time, but I'd use Amethyst if it had less data usage because I prefer native apps. Thanks for your tips🙏