The tank is heavily planted. I dont want to crush the plants. I'm not having problems, i just wanted to replace the rocks with driftwood or more plants
1) use a pitcher. I assume you're using buckets since the sink isn't hooked up. Still do that, but dunk a pitcher in the bucket then pour into tank, using your hand to shield the flow. Will take longer, but all you need extra is a pitcher
2) make a refill tool. If you decide on using the sink adapter later on, or if you ever use a power head to pump water back in, this tool will work. Basically, you just cap off the bottom of the siphon and drill holes in the lower area of the siphon cylinder. Turns Jet mode
into Shower mode
on the refill. Tweak it to customize to your needs, like size and amount of holes, secure the hose end and cap end so it stays together, etc. Can be as simple as modifying your Aqueon siphon, or make one from plumbing parts from the store. Can even make it with a J bend to hang on your tank for hands free filling.
3) get a shower caddy. The thing folks use to plant stuff higher in the tank, or have terrestrial plants above the surface of the water. You could strategically place a shower caddy inside the front or side wall (via its suction cups) and pour your water bucket over that to block the flow. Like and extra hand in the tank...
4) use a step ladder. If you have the room and supplies, place a step ladder next to the tank and elevate your water buckets higher than the tank. Now you can use a siphon hose to fill the tank slowly. Depending on the diameter of hose and height above aquarium this could still disturb your substrate. Make sure flow isn't too strong and/or clamp the hose to hold it parallel to substrate.
Good Luck, Hopefully something in there was helpful!