I assumed I just hadnt gotten to that point in the story yet, but after beating the game, Im still left scratching my head a bit
In short: All cards released on MTG Arena are legal in Historic, except those that were banned from the format or cards that werent ever legal in the first place
Regrettably, Tithe is on the Reserved List, so it is ridiculously expensive for the effect you get, but if you want an inexpensive alternative that is slightly watered down, Gift of Estates plays reasonably well, trading instant speed and doubling its mana cost for a guaranteed 3 plains cards
Lands, mana rocks, tutors, protection spells, powerful legendary creatures, and splashy finishers are all categories worth checking
It's worth noting, though, that usually these high values do eventually drop, whether or not the cards actually receive a reprint
These all bring value, but with Shimmer Dragon on the field, each of them also represents half of a card-draw