Thanks - my memory isn't what it was so I did wonder if I'd plucked Hagel / Hagen out of thin air!

Ludendorff really wanted his Flanders offensive as first option - presumably Hagen was the name assigned to it later in 1918 - but went with the attack on 5th Army as the ground in Flanders hadn't dried out (which wasn't really unexpected given the problems the British experienced at Passchendaele. Presumably he though beating the British straight on was the best option, maybe followed by a shorter advance through Hazebrouck to the Channel coast.
Hagen was the name assigned to the follow up to George. George couldn't be launched quickly enough, so a modified version was created and even that proved too problematic, so Georgette was created as the rapid follow up. After Georgette was conceived, a follow up named Hagen was planned to exploit the opportunities created by Georgette, but would require time before it could be launched, as it was planned for on ground not yet taken or integrated into the logistical system. So Hagen was a plan that waited to be planned until Georgette petered out so that it could then determine the ground it had to operate on.
In terms of the Michael-Georgette situation, it wasn't the dryness of the ground that was the main issue, though Kuhl, one of his staff officers, did favor it. Ludendorff thought there were far too many problems with the George plan, including lack of decisiveness. The risk of a late rains in May making the ground a problem was but one factor. As it was Lossberg was claiming for a wait for attack into May, so there wasn't a consensus that waiting was a problem. Michael was just the one operation of several developed that didn't have an issue due to the weather and offered decisive results, which would then assist the follow on operations. Since it could be launched earlier than the others because weather was less of an issue, the process could get going before the American threat built up.
This is all from the planning chapter in Zabecki.