Local "point of use" hot water heater
The wait time to get hot water from a tankless heater is very common, especially when installed some distance from the faucet. The specs for different heaters varies as far as the volume needed to trigger to on. Most do not require full flow volume to turn on.

There are a few small point of use tank heaters, such as American Water Heater Company's Titan series (Lowes) that only store 2.5 gals of water and operate on 120VAC. You should have a separate 120 volt circuit for this type of heater however.

Keep in mind, 2.5 gals is a lot of water in one use, unless you tend to run hot water for longer periods of time. Your main tankless will deliver hot water to the small tank heater by the time temps drop in the small tank, theoretically the small heater will not need to turn on and the hot water will simply pass through.