Well I found one problem.
We installed the reactlog package via remotes::install_github(), and our git not having LFS present meant the reactlog.js file downloaded and installed by install_github() was a stub meant for git LFS:
version https://git-lfs.github.com/spec/v1
oid sha256:7b7a2271d7aa4350090323c4031843b60190e5f52557b545a960bc165e450e92
size 710689
and not the actual javascript file. This of course shows up as a javascript syntax error and why the render page is almost blank. We're going to install from CRAN i.e. from a tarball with these LFS assets baked in and hopefully problem fixed for me.