Tip: Cheap fix for the leaky heat exchanger
Is your Niro blowing coolant out the exhaust? That heat exchanger in the catalytic converter was a bad design idea. Exhaust gases are acidic and highly corrosive. I believe this is the reason the heat exchanger leaks -- it develops pinhole leaks from corrosion.
Bypassing the heat exchanger is a well documented fix that I won't cover here. It's not a bad idea and it's my fallback plan.
But first, try adding a bottle of head gasket sealant to your coolant reservoir. I used K-Seal and it has been holding strong so far. These sealants are designed to stay a liquid until they come in contact with exhaust gasses. The exhaust gas hardens the sealant into a ceramic. It's a dubious fix for a head gasket leak. Honestly, it's a dubious fix in general - but our heat exchangers happen to be the perfect application of the stuff. The leak is sitting in a stream of hot exhaust gas.
I've heard stories of these "magic liquids" ruining engines. I think they're harmless here, for two reasons. First, I use the version that hardens when it meets exhaust gas. Don't use the sealants that start out thick and goopy, because they run a higher risk of blocking a coolant channel. Second, I'm assuming that you don't actually have a blown head gasket. This means your coolant system is still self-contained and isolated, so there is no risk of hardened sealant entering a combustion chamber and saying hi to your valves.
Will this last indefinitely? No, probably not. But it should last a few years. It will last longer if you add a fresh bottle every time you flush your engine coolant. If it stops working, you can still replace or bypass the heat exchanger.