[exim] converting from debian package to source

My mail servers run, and have run for decades, on Debian, and I've
always used the Debian package for exim4, though I don't use debconf
for my own additions, but just edit the conf.template file as if it
were a .conf file.

The pain of dealing with Debian's antiquated versions (4.92) and
gratuitous messing around with upstream's configuration (most recent
annoyance, not supporting built-in SPF) is prompting me to think about
switching to using the primary source.

I wonder if anybody on this list has done such a conversion recently,
and would have time to share the chief gotchas they encountered.

If you reply to me, I will summarize to the list.

Thanks,
Julian.