We are planning on installing 2.25" solid oak plank 3/8" thick with glue in an 11'x12" bedroom . The problem is the slab has settled causing a big crack in the center, it seems the highest spot in the room is where the crack is. From the crack to the baseboard there is about a 1/4" drop on either side. Am I going to have to build up the subfloor all around the baseboard to be level with the crack? or should I scrap the idea of glue and go with a plywood subfloor and then staple down the plank. Is there an easier way that I am blissfully unaware of that someone can guide me with? Thanks for any advice you may be able to give

Grind the highs and fill the low spots.

If the back of your home is falling off, it should be visable in the rest of the framing/sheetrock, with cracks and doors not shutting properly.

1/4" is not that much.. easily fixed. Hire a pro to do the job. The crack is bothersome, more than the Flatness.
Yo may also need to seal the concrete.