[dcb]

best fuel economy and best bsfc peak are two different things, and the electrical demands (equipment costs) will go up very much if you expect to accelerate anything car sized up to 35mph.

If you accelerate and shift properly (centered around bsfc parameters) your engine will be extracting the most energy from the fuel you give it. Whereas with cruise you are probably far from bsfc peak already.

So with your limited battery capacity/money mode I think that is your best bet is to try to help the engine when it is not running efficiently. The batteries you have will run longer too if you only maintain a given output rather than peak them with acceleration demands, and you can optimize (gear) the system for a target speed. plus the controller then only really needs to be an on-off switch, basically.

So for keeping the engine in it's most efficient mode and the electric (with limited capacity) in its most efficient mode, add electric power to cruise, IMHO (and freewheel AND braking regen if you can swing it).