Hey there! Always nice to see a miller about
There are a few cards I would suggest.
Glimpse the Unthinkable,
Mind Funeral,
Archive Trap - really potent mill cards (though I understand if Glimpse is out of budget range)
Fraying Sanity - not out yet (2 weeks!), but seems strong in a mill deck.
Crypt Incursion - one of the weaknesses of mill decks is getting killed before you can mill them out. This helps significantly with that.
Surgical Extraction,
Extirpate - very nice in modern to get rid of combos (can also hit nonbasic lands, so basically annihilates tron unless they're very lucky)
Startled Awake - very slow, but curves nicely with
Fraying Sanity, and is repeatable.
Chronic Flooding - I just like this card
it also mills
Oh also
Nephalia Drownyard...also slow but is repeatable mill that doesn't cost you a spell slot.
Anyway hope it helps