OP here. Thanks for the tips guys.
I've got it all built out. In another thread I mentioned that a combination of time, money and baby has put a damper on some of the initial ambitions I had - so I built it mostly stock with plans to work these upgrades in in the future.
A few take aways I've had - the melamine isn't as bad as I thought it was going to be. It's still going to be slowly phased out as I can afford to, but after some of the horror stories I've read here and elsewhere, I thought I was going to be working with something that had the consistency of wet cardboard. I've been careful to not torque the nuts too hard, to keep it from deforming. I tend to check the corners every day or so, just to make sure no vibrations have wriggled anything loose, which has happened. Looks like there's a rubber mat in my future to keep the vibrations down, too.
I'm slowly calibrating the system. I don't see any immediate problems with the x/y idler, but I'll keep an eye on them. However that stock Y-endstop seems as if it was an afterthought, at best. Off to Thingiverse I go.