Moving houses, need opinions on floor bracing

[Frogger]

In order for your tank to drain you need a minimum of 1 to 2% slope over the entire run, there can be no dips or it will clog. A 1" and 3/4" bulkhead will not move much water at a 1% grade.
Increase the grade or increase the size of the bulkhead and increase the amount of water that will move through the line. You can always adjust the size of your return pump to meet the amount of water your drain will move. You do not have to have a large turnover in your sump to make the sump effective.

I am not an engineer but bracing a tank is not a bad idea. You would likely need a footing in to hard pan to work over the long run. A large 2'x2' concrete pad on the soil surface might work for a short period, but could be affected by the changes in soil moisture. You do not want your floor going up and down.