A new carpet rule to disable pickaxe breaking speed on minecraft:budding_amethyst that would let you break amethyst buds without being scared of accidentally breaking budding amethyst block

You could use a data pack to remove block minecraft:budding_amethyst from tag #minecraft:mineable/pickaxe, but by default, doing that would also need to reset the contents of the tag back to the default list (minus the aforementioned block), which may be unwanted for some players.

yeah, datapackable

also 100% scarpetappable, so no need for custom rule. Feel free to request / propose such an app to be added to the appstore.

Scarpet app one-liner that implements what you're looking for:

Method 1: /script run command:

/script run __on_tick() -> (player = player();if(player~'trace' == query(player, 'trace', 4.5, 'blocks') && player~'trace' == 'budding_amethyst',modify(player, 'effect', 'mining_fatigue', 1, 2, false, false)));

Method 2: Standalone app (create an app with the following code and load it):

__on_tick() -> (
	player = player();
	if(player~'trace' == query(player, 'trace', 4.5, 'blocks') && player~'trace' == 'budding_amethyst',
		modify(player, 'effect', 'mining_fatigue', 5, 2, false, false)
	)
);