Are the changes from 18.104.22.168 that much different than .5.3?
(IE: should I put off playing DQ until later or not?)
The only major change you're likely to notice is that there's now an achievement interface. The achievements were always there, but you could only see them on Steam. In the current version your achievements are still tracked as normal but you just can't see them. There's a few minor things as well, and some improved mod support, but that's the "big" one. (Oh, and Ether Flash dispelling darkness 100% at rank 1.)
There's no reason to wait on buying it, though, honestly. It's a great game and the only likely changes from this point forward are non-gameplay ones.