(Played early morning the next day again.) After first finding some Double Fanucci cards I put them in my stacks, using only one suit per stack, because I didn't know any better. Then I read
this thread which states: "Never combine three or four different sets (0% relationship suits).", causing me to put two different suits (or sets) in my stacks, but only two. Although this got my characters much better stats, the part where it says (the whole sentence really) not to mix more than two suits is apparently bad advice!
I decided to look into Double Fanucci a bit, and found
this page. It actually suggests mixing more suits, and that turned out to be much better advice. Using this tool my characters' stats improved once again. Not by much; the benefit would've been much greater if I knew about this earlier. Still, they now have a total attack of 665, and defense of 870. (572 for Retodon8 due to slightly better armour.)
L27 Retodon8 explored L26 Peltoid Valley. He levelled up to L28. There is now 655k in zorkmids in his bank account, and he owns 81 cards.
L31 Denroot explored L29 Peltoid Valley. She has 565k zm in her account, and she owns 88 cards.