Oh, man. That's not good.
I thought for a second to initiate rsync job from the remote host but I remembered about this one time rsync on a target macOS was too old (no surprise there) and it failed one of the options—so… it's required on both ends.
Thanks again everyone. I'll go with scp, it has to be there. The good thing is that it's only a handful of files that have to be overwritten anyway triggered by a successful task, e.g; successful cert renewal. There is no need to compare files or anything like that.
I'd like to share it when I'm done (almost there, as soon as I simplify error checking) so I don't want to script an install because I think that would be too invasive.