When I'm at that point of the game (before Noel's badge), I got a butt load of balls and money to spare. The NPCs at the Opal Hall give you lots of money. You can even use a Frisk mon to check. I thought of and tried Thief but it's like only 1 out of 5 times I met a Luvdisc without heart scale
fishing is not frustrating in this game at all +.+ you can just slow the game down when it goes ". . ." and toggle it up if u caught something