Update:
It appears that my problem was with DG employee knowledge, or a lack thereof.
I still don’t know what the limit is per transaction, but it’s at least $500 around here.
I went to another DG today and asked to load $500 to my card. I’d have tried more, but I was down to my last card. Cashier had no problem completing the transaction. She had to swipe my Serve card on her machine, but then I used my gebit on the normal customer machine.
I’m debating if it’s worth going back to the two close to my house and asking them to attempt the transaction. They seemed friendly enough, just not very knowledgeable.