Here's a SUGGESTION for learning to become proficient in transposing & playing a song in other keys.

Start by taking a simple song that you know real well by memory such as: "Twinkle Twinkle Little Star" or "Row Row Row Your Boat". Start by playing the song in the key you know it best in, such as key of C. Then learn to play it (by ear) in C# which is 1/2 (semitone) up from C. Once you master this, learn it in D, and so on to cover all 12 keys. Soon,thru practice, you'll eventually develop your ears & playing skills to transpose songs on the spot to any key. The benefits and rewards will pay off in your improved keyboard proficiency.

I suggest first beginning by just playing & transposing the melody, and then adding transposed left hand chords (with auto accompaniment) after you've gained confidence & proficiency playing the melody.

Scott