Suppose we have our spacecraft in a circular orbit around the Moon and we want to transfer it to an elliptic orbit with a medium-thrust (10-100N) retrograde burn. Based on the current state estimate, we want to determine a propellant-optimal steering strategy to achieve the transfer. The thrusters operate at full throttle when they're activated, and thrust levels decline with propellant consumption (the propulsion system operates in blow-down mode).
It seems to me that it is best to consider such a problem as a two-point boundary value problem, with the initial and final state vector given by some set of non-singular orbital elements (such as the equinoctial elements), and the dynamics given by some accordingly modified form of the Gauss planetary equations. Are there any standard and/or proven methods to solve this problem in real-time, aboard the spacecraft? Or are there other common guidance strategies that are better recommended?