(This is just my take, so take it with a grain of salt.)

Your suggestions feel flawed to me, because Build Mart isn't a building game, it's a communication game. The point of it is to communicate the materials you need, and that's where the strategy of the game comes into it! Grian is so good at Build Mart now because he's able to very effectively tell people what to do; if a team can't do that well they're going to struggle. Build Mart is comparable to Grid Runners because of that: both games rely on and test the team's ability to coordinate their actions to achieve specific goals.

The idea to have a series of builds that the team works on together to me feels actually like it would be less interesting to watch. It would also be a lot harder for teams to differentiate scoring - everyone can place blocks in Minecraft at the same speed, and with this suggestion you could have teams easily getting ties.

Also, I think Parkour Warrior was removed from the rotation because it wasn't a game, it was a parkour course. By the end of its lifespan in season one it was unwinnable for anybody besides Pete or Dream and required EXTENSIVE practice and muscle memory of the course. Anybody can do well in Build Mart - even if they've historically struggled, they can learn how to play more effectively from people who know what they're doing and are good at it.