This is a question mainly for more advanced pianists: What is a better way for me to compose? As of now, I compose mostly in C or A, as I'm not as familiar with other keys in terms of finding chords immediately (esp. inversions, diminished, 7ths); however, I want to play a recent piece in a different key — I already have the ideas but only in the key of C, and if I transpose, some fingerings may potential feel rough to play, and tbh, it takes time... May I have some suggestions for my future compositions? Thanks for any help.
Most effective, and simplest in the end, is to learn the other keys, playing scales, chords and arpeggios much as you already do in C or A. Would that be A minor?
If you do it gradually, and in an organised manner, it will be fairly painless. In C, it's all white keys - unless you modulate. Use G major as a start point - there's only one difference with no white F, but an F# instead, and things gravitate to G rather than your normal C.
Then you either carry on with # keys, adding C# note to be in D. Or - you explore the b, in key F. This will incorporate Bb instead of B, and the home note/chord becomes F, not the usual C. So, work through the 'circle of fifths'.
You will find more inspiration in other keys, if not only because of the different fingerings, which make some changes easier, some a little more tricky, but different nevertheless.
If you want to write music specifically for the piano that will be satisfying to play then you're much better off writing it in the key you want it to be in. Blindly transposing from one key to another can make a piece much harder to play if you're not careful. It's not always the "harder" seeming keys that are more difficult to play in; keys with lots of sharps or lots of flats can actually make some chords/arpeggios/intervals easier to play.
A good example of this is Schubert's Gb Major impromptu (D899 no. 3). One of the early publishers of the piece decided to transpose a semitone to the "easier" key of G major, presumably in the hope of increasing sales. After all, who wants six flats if they can have one sharp? However, in Gb major the left hand part sits nicely on the black notes and feels very natural to play while in G it's just awkward and difficult to play smoothly (in my opinion).
As Tim says, you really need to learn more scales, but that's no bad thing. You'll get a lot out of it both as a player and a composer.
If you're writing specifically pianistic music, hand shapes come into it. Gestures that lie well under the fingers in one key may be awkward in another.
If you're writing songs, key will be largely determined by the singer's vocal range. Cope.
You should practice playing existing pieces in all keys. Then your fingers WILL find the chords more easily.
As Tim wrote, first be comfortable with different diminished, half-diminished, augmented, and major/minor chords for all keys first through practice exercises.
Then, if you would like to delve into the world of complex music, try composing keyless. This does NOT mean composing in no key and producing random notes. Instead, do not write any key on the music score. Start by composing in any key that comes to mind when thinking of a motif (like an idea one could develop). It could start in simple keys like C or G, or more advanced keys like C# and Ab, but the starting key does not matter. Obviously, don't start with the melody (that comes later)
As you continue to compose in the key you started with, don't let that key restrict you. Float between different keys, and don't try to establish a single key as you modulate through different ones.
After some practice, you should start veering away from standard chords like C Major, or a diminished chord. Instead, try to stay in the grey area between tonality and atonality. One could do this by using dual tonality (two chords at once), standard chords plus a different note, etc. Try experimenting with unique note combinations to see which "chords" suit the needs of your piece.
Just remember not to slip into tonality (always reverting to standard chords and one key), or go into complete atonality (no tonal center at all, all dissonant notes), but remain in the grey area as you continue to explore and develop your style.