I resolved this myself with a search that turned up a 2018 post from Mark that suggested typing "mailman aliases" to create the hash/alias/lookup table/file things in /var/lib/mailman3/data (that's where they are on Ubuntu 18.04). I looked in that folder and sure enough it was empty so I typed "sudo mailman aliases" which did indeed create the 4 things that needed to be there: postfix_domans postfix_domains.db postfix_lmtp postfix_lmtp.db
But, they were owned by root:root and everything else under that /ver/lib/mailman3 folder is owned by "list" so I took a guess changed the ownership on the 4 files to list:list.
Seems to have worked! I was perplexed about why I had to do this though because these were created automatically last time I did this, but I figured that out too. There is a manual step with this install (mailman3-full on Ubuntu 18.04) that involves added 4 lines to the end of your postfix config file (/etc/postfix/main.cf). Those 4 lines are tricky because 2 of them are long and wrap in my documentation. I had a typo probably introduced by copy and paste, whereby 2 of the lines were on the same line and yes, those are the lines that specify where the hash tables go.
So - lesson learned, follow your own instructions :-) I'm pretty sure that had i set those lines up properly the hash files would have been set up properly when mailman first tried to initialize. Maybe I'll try this again one day and see...
~Kelly