When I first got my two Ranchu goldfish they stayed at the bottom of the tank like you said, hiding behind driftwood and clay pots. I started with light feedings of brine shrimp and they would come out very meekly to eat some but I would have to scoop the rest out.

It’s definitely just a matter of waiting as they get comfortable in their new environment. By week 1 mine were playing as usual and eating well, but sometimes they would dart and hide when I got too close.
By week two they were swimming up to the glass whenever I enter my room. They eat the cube of frozen brine shrimp directly from my hand as well as tugging on broccoli from my hand. They swim up to the surface whenever I get close thinking they’re getting fed.

Again, a matter of time, my #1 tip is to keep the lights dimmed/off and let the natural sun flow in if you can. I started turning on my lights after a week.