Yeah, $5 definitely isn't a whale. I hope that's the monthly subscription, which is the right way to spend that $5. The longer you've played, the less luck it takes to have high star UR generals. A three star that fast on only $5 is pretty lucky, though.

A whale can get the high star UR generals much faster because, for example, with last week's "koban kickback" event, if you drop something like $150 on the game and then spend 20000 Koban, it would give you, among other things, a jade ticket, two universal EVO cards, and a UR EVO card. That's a three-star UR general right there. Few people are willing to drop that sort of money on the game, though.

As to the substance of your comment, ranking up accessories at the blacksmith is far more important than ranking up weapons and armor. For Sanada Yukimaru in particular, it's not the reflect damage that is the problem. It's that your party is simply too weak. A stronger party would let you blast through and ignore the reflect damage.

When you don't need an A grade or better, about the only reason to worry about corsair reflect damage is if you have a very high single-target damage general (basically Tiger of Kai or Sanada Yukitaka) and don't want to waste a bunch of troops by doing five times as much damage as your target has in troops. I pull my Tiger of Kai out of the lineup when fighting some weaker opponents just to avoid that. That's not your problem yet.

The other notable PVE case where reflect damage is a problem is the Uesugi Kenshin opponent on the Saturday dailies. That's due to Uesugi Kenshin's skill, which all five of the generals have. To handle that, I have to swap some generals between my A team and my B team to go in with just tanks and archers, and not do that much single-target damage.

Handling reflect damage in PVP is a different beast entirely. You don't seem especially worried about that yet.