I think it’s a good idea to think about replacing some things before they fail, such as a hot water heater. The hot water heater can fail catastrophically, which causes water to leak out of the bottom of the water heater quickly, creating a flooding problem, which can damage your house. This could be especially bad if that happens when your house is unattended for a long time.
The downside to tankless water heaters is that they are more expensive, and the hot water outage is immediate, if it’s an electric tankless water heater, and you have a power outage, and don’t have a electric generator.
Our central AC failed in Minnesota in the middle of the summer, and had to be replaced. At the same time, we replaced the furnace because it was about 20 years old, and had to be repaired two years in a row, both were emergency service, which is more expensive, in the middle of the winter. We wanted a more reliable HVAC system before we started snow birding and doing more traveling in our upcoming retirement years.
Having a split AC system in addition to your central AC makes sense. You could have a split AC system in the master bedroom and lani. If the main AC system fails, you have a backup AC system. Plus, you could keep the master bedroom cooler, run the split system only on less hot nights. Or, run the lani split system only when you use the lani.
Here’s a link to the Honeywell mixing valve. It can increase the capacity of a water heater by mixing the cold and hot water after the water heater. Less cold water enters the hot water heater, which increase the capacity of the hot water heater.
honeywelll mixing valve increased hot water capacity - Google Search