Is getReserveData() on the LendingPool contract the correct way to find the current APY to borrow?

Calling getReservesData() or getting the rate from graphql will give the APR in ray units (27 decimals). To convert APR to APY the formula is

((APR/secondsPerYear + 1)^secondsPerYear) -1

or you can use the formatters in aave-js or aave-utilities to plug in the reserve data and formatReserve() will return both the APR and APY.