You bought the house with ready hot and cold water pipings for your solar water heaters (can be used also for storage water heaters), which is a luxury compared to only cold water piping (which you can only use with instant water heaters).

Hence why not just use the hot and cold water pipings rather than letting it go to waste? If u don't want to use the solar water heaters, maybe convert to using storage water heaters instead? A 90L storage water heater would do, such as this CENTON Neptune (Shopee link - https://s.shopee.com.my/9zhBPf4sNC).

Do you have a bathtub in your house? If yes, then no brainer, just continue using either your existing solar water heater or change to a storage water heater. Instant water heater will not be able to support bathtub usage (not hot enough).