Is this rule as inflexible as it could be applied or are we merely trying to think about different categories of cards to make our decks less unfun to play against but still effective (its the second one, by the wayI cant say that enough)
If our opponent counters, Thoughtseizes, or destroys combo pieces like Altar of Dementia or Thran Vigil, we can always use Lurrus of the Dream-Den to play them from our graveyard
This is exactly what the Legacy Aluren deck aims to do
Personally for me, this makes for somewhat uninteresting gameplay to watch, and the subgame of who gets to resolve W6 first is quite frankly a little boring
Tutors In a 99-card singleton format, tutors are a powerful way to add consistency to your decks
Further Upgrades The first cards I'd pick up to further juice up the deck would be untapped mana-fixing lands: Watery Grave, Shipwreck Marsh, Underground River, Morphic Pool, Darkslick Shores, Polluted Delta, Sunken Ruins, Undercity Sewers, Urborg, Tomb of Yawgmoth and then maybe a couple more Island / Swamp fetches