Another few hacks that have improved and perhaps fixed this, at least so far.

Doubtless, this is a memory management issue requiring both guest and host tweaks.

I got a big improvement with two guest tweaks, first by manually defining pagefile.sys size in System Properties (it was system-managed and >10GB; I reduced it to ~2GB and min=max), and second, by reconfiguring superfetch on the Windows 10 guest for boot only, not boot and applications.  This prevents superfetch from loading all of one's commonly-used apps into memory in advance, substantially reducing dirty pages.  So:

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management\PrefetchParameters]
"EnableSuperfetch"=dword:00000002

Meanwhile, on the Linux host, and in addition to the foregoing IOMMU tweaks, I've set the following kernel parameters in sysctl.conf:

vm.compaction_proactiveness=0
vm.swappiness=1

These two parameter settings may be overkill in light of other kernel parameters that configure memory compaction and memory reclaim but (a) they work; and (b) I don't have time to figure out the other parameters.