The long game with Belcher isn't long per se. But from my experience testing the grind plan in PSI, being able to still be very much in the game 10 turns into the game, even with Counterbalance, Top, Jace, Liliana, with an opponent holding several countermagic. I've won through that exact board position many times. I think its time we started porting some of this strategy over to Belcher. Carpet of Flowers... is absolutely nuts in the mid to long game. Belcher also needs a lot of time to rebuild so having a Carpet and forcing the blue player to sandbag you on lands allows you more time to rebuild, meaning you can land Empty the Warrens, or try to land a Belcher with some protection spell.
Go off or bust is a strategy that works when racing bears and other lock pieces. You need a contingency plan against blue. They don't often play lock pieces so you'll ideally have time to assemble multiple attempts or one solid attempt with Empty the Warrens, especially if they don't expect Carpet of Flowers, or sit on fetches as you acquire more IMS's and artifacts that you can land on the board and sculpt your hand. The long Belcher gets to sculpt its hand, the greater chance the U player has of running into Empty the Warrens instead of something they can counter.
There's always Flusterstorm, which is run quite frequently in the sideboard. Autumn's Veil plays around it rather nicely but Flusterstorm doesn't hit Belcher. This means if Veil is protecting Belcher rather than Empty the Warrens, then your opponent will have to choose wisely with their countermagic. Holding onto Flusterstorm in fear of Empty the Warrens could lead to a resolved Belcher. Keeping Probe in post-board would likely help facilitate this.

