The key card is present but its support is thin.
Prioritize useful support such as Goobbue Gardener or Town Greeter.
Gardener accelerates Horizon, which finds differently named lands and can bring Sentinel toward seven lands. Greeter finds a land to hand while Ignis can put one directly onto the battlefield.
Your deck already enables the plan but is losing the early turns.
Take a castable early creature or efficient answer before a second expensive payoff.
Town Adventures still need a normal land play afterward. A colored Adventure cost does not make the land side cost mana, and mana artifacts do not count toward Sentinel’s seven lands.
The support is missing
Favor independently useful creatures and interaction if the narrow support does not arrive.