Ah, I found the problem. The modules were enabled in fpm after all, but I forgot to disable the old php version in apache and I think mbstring and curl are incompatible with Ubuntu 18 and were uninstalled in the upgrade.
So, while php 7.2 was enabled in apache, so was php 7.1, which it defaulted to–perhaps configurable in apache config, I’m not familiar.
I ran the following:
sudo a2dismod php7.1
And that worked for me.
Thanks @murrant. Your reply helped get me searching in the right direction of what to check for.