First of all, do not use plug + socket connection to power up your water heater. Is it not safe and definitely against SIRIM & Suruhanjaya Tenaga's guideline. Suggest u to change to a Water Heater Connector like the one shown below. U can get it easily from online platforms, example here - Shopee / Lazada


If u want stronger pump, get AC Pump as compared to DC Pump. Noisier, use more electricity, lesser warranty period, but slightly stronger than DC Pump. But instant water heaters' built-in pumps, regardless of whichever brands or AC / DC Pump, are still small, so don't have high expectation on the additional water pressure boost. Also, many brands like Panasonic. Joven, Midea, Toshiba (I think) are phasing or have phased out AC Pump. Lowest price instant water heaters with AC Pump should be Centon, like this one - Shopee / Lazada
U will need at least 2 litres / minute water flow for most instant water heaters to turn on as instant water heaters use flow switch as a safety measure (low or no water flow, instant water heaters won't turn on to prevent the heating element from burning). So if your water flow is extremely low (lower than 2 litres / minute), instant water heaters won't turn on even if they have a built-in pump, unless the instant water heaters have the Pump Activator button to manually jump start the pump so that water flow can be increased enough to turn on the instant water heaters' flow switch. I think so far I saw Centon and Alpha brand has this feature, such as this Atlantis model - Shopee / Lazada
But ultimately, getting an external booster pump like Grundfos will be the best choice, as u can use it to boost water pressure throughout the house, not just for shower, example here - Shopee. Downside is it is much more expensive, and with the higher water pressure generated, u might need a higher power rating instant water heater to generate enough hot water (higher water flow = lower output water temperature).