Take any block that usually instamines such as crops and slime blocks. Then take a tool with a block break speed add multiplied total modifier of 0, or a tool component with default mining speed 0, and it correctly makes all blocks unbreakable. However, if the value is above zero, no matter how close to zero, any block that normally breaks instantly, will break instantly. There should be a way to make instant break blocks break slower, like all other blocks. There is no in-between for blocks like crops and slime blocks, you either can't break them at all or instantly break them in a jiffy.

Tool command examples:

/give @p iron_hoe[attribute_modifiers=[{type:"player.block_break_speed",name:"player.block_break_speed",amount:-0.999,operation:"add_multiplied_total",uuid:[I;-1653246257,-1376432704,-1932170708,1515677938],slot:"mainhand"}]] 1

/give @p iron_hoe[tool={default_mining_speed:0,rules:[{speed:0.001,correct_for_drops:true,blocks:["wheat","slime_block"]}]}] 1