If you are concerned about getting some extra MQMs by the end of the year, looking into the Delta Reserve credit card may be a good route -- it's expensive, but the purchase-MQM prices they've offered the past few years have been extortionate.

It's not entirely clear what you are planning to do with the M fare but in most markets buying an M fare is going to be expensive (often more expensive than discounted/sale business class fares), so if you are planning to do a mileage run to top up your MQMs by the end of the year, trying to do it on an M fare will be a very expensive way to go. If you are planning to skip Gold status and just use miles to upgrade your trips next year, that will also be expensive, but might be tolerable depending who's paying for your trips.

Upgrades are also not guaranteed -- even if you buy an M fare and use miles, you can only get the upgrade confirmed if Delta allows upgrade space. You can waitlist and your upgrade may clear at boarding, but in that case you would have a coach boarding pass until just before boarding the plane, and would therefore not get lounge access (unless you had Gold status or higher). If CDG is of particular interest, for whatever reason day-of upgrade waitlisting is not available in AMS, CDG and BOM, so in those cities if your upgrade didn't clear in advance then there would be no chance of clearing at the gate.