1. Is there a way to add a vanilla tag (like the /tags command ones) to a modifier?
2. how do you set attributes to stack/add instead of replace when multiple are used at once?
   (Working with datapacks for a semi race system using modifiers)

as an example for the 1st one, being able to execute a command as every entity with the “magical” tag or having a spell check for a tag and run a specific set of actions based in the result (currently using that with passive spells for things like silver or garlic type effects)
There is a tags section in the mage data but i haven’t been able to make them compatible with selections using @A[tag] commands or when checking using the/tag get command

You can specify an operation for attributes, like:

damageboost:
  entity_attributes:
    generic_attack_damage:
      value: 20
      operation: MULTIPLY_SCALAR_1

Options are from here: https://hub.spigotmc.org/javadocs/spigot/org/bukkit/attribute/AttributeModifier.Operation.html

I have no tag support, not sure that's even possible outside of a datapack.