Yes indeed rho is 0.3%, i must have mistyped, and yes agree with you assessment re the need for revenue from transactions to pick to to maintain the same level of return for pool operators.
As to why the formula is the way it is you might want to check pages 34 onwards in the formal specifications doc link1 and academic paper link2
On the back of this I have created a Calculator to simulate pool Rewards by changing any of the pool and network parameters. Can you take a look here and let me know what to you think?