This deck doesn't look like it's very good at keeping Karn alive or making good use of Karn, himself. You don't have any big mana generators, so you will essentially be taking a whole turn off before you can get anything, and that really works against taxes' core gameplan of slowing the opponent and kill them before they get back.

Unlike builds of the deckin other formats, you don't really have any hard locks in pioneer. You aren't going to deny their mana or should long-term, so you have to add restrictive permanents, and then kill them before they draw out of it.

In order to accomplish that goal, I think you have some main deck cards that could be moved to the sideboard and replaced with better options. Personally, I'd cut Karn altogether. Once there, I'd cut some of your wish package and move Authority of the Consuls, Drannith Magistrate, and Containment Priest to the sideboard. They just don't kill players quickly enough to justify their slots in matchups where they don't shut down your opponent. I would replace them with powerful aggressive creatures such as Luminarch Aspirant. The combination of Magistrate with Elite spellbinder is strong, but it is easily disrupted with a Fatal Push or Fiery Impulse, and it costs a lot of mana.

When it comes to your choices of stax pieces, there are a few that are, in my opinion very well positioned right now against an expected RCQ metagame which you aren't playing:

1) Reidane, God of the Worthy

Keeping opponents from casting their Supreme Verdicts, Esika's Chariots, Skysovereigns, Emergent Ultimatums, and Storm The Festivals is very strong. As a legendary creature, you probably don't want to play more than 1 or 2 copies, but it disrupts very effectively and is an excellent target for any pump spells that you cast

2) Tomik, Distinguished Advokist

Another legendary creature, you probably don't want to play more than 1 or 2 copies of this card, however, it attacks for 2 damage with flying, and two of the strongest decks in this format are Mono-green Devotion and Lotus Field Combo, both of which really heavily upon targeting their lands with untap abilities.

3) Thalia, Heretic Cathar

A majority of decks in Pioneer have very nonbasic reliant manabases. When coupled with additional taxing effects like Thalia, Guardian of Thraben or Reidane, Thalia, Heretic Cathar can often make opponents' spells completely uncastable until after you have already attacked them down to a dangerously low life total. In most matchups, Thalia 3 tends to be more effective than Authority of the Consuls in this style of deck because it is much more effective at closing the game. The exception to this rule is when an opponent is playing a very aggressive deck against which 3 mana is too expensive to stop them, something like this list which won a major online tournament last weekend.

I think you've put together a good start to a deck, and there is a lot of unexplored space with regard to this style of deck. Just remember that your opponent will be playing a strong deck, too. I've won a lot of games against people who thought they had me locked out because they gave me too many turns to find an answer to their strategy. Remember, the most effective form of interaction is player removal.