Thanks for your replies, everyone.
As for why I "choose" to stick with GLM - I have zero experience with GAM,
and it's a time-sensitive analysis, so there's just a time x experience
issue.
It sounds like maybe the glmer() way is not the way to go...
I'm also exploring the SpatialFilter approach to getting rid of nuisance
spatial autocorrelation, but am running into computational issues - it
_really_ doesn't like my dataset.
If anyone on here has any experience, input would be welcome. I have
already posted a SpatialFilter-specific question on the r-sig-geo mailing
list (https://stat.ethz.ch/pipermail/r-sig-geo/2018-January/026282.html),
although I managed to provide a toy dataset with a mistake in it.
Either way, thank you! At least I can stop digging in this direction.