That's weird about the two-card situation..... For some reason I never got around to putting a card in the second slot in my A7R IV and have had no problems in the two and a half years that I've had that camera. With the A1 I realized that I'd be doing a lot more shooting in situations where I'd really appreciate having a second card in there ready to continue on with wherever the first one left off, so set that up fairly soon after I got the camera. I haven't tried using it without a card in the second slot; should experiment with that, see what happens....
Even when shooting around the lake I always carry a small pouch with an extra battery and extra card in it; more than once I would be out with the A7R IV, all the way around the lake and would use up the card or, more often, the battery would need to be replaced and so I'd have to come all the way back home. Didn't take long to train myself to get into the habit of checking the remaining battery capacity before leaving the house, even when just going around the neighborhood.
At home, after I have run the card(s) through the card reader, I check to be sure everything looks OK and then, whether or not I'm going to begin post-processing immediately, I take the card(s) right back to the camera, which I have deliberately left out on the table after putting away the lens(es) that I've been using, as this serves as a reminder that the camera needs to have one or more cards replaced in the slots. I then go ahead and put the card(s) back in, format, and only after that do I put the camera body away in the cabinet.