Just a couple of thoughts about keys on the guitar:

Except for the issue of open strings and easy-to-finger chords, there's not much difference in difficulty between playing in different keys on the guitar. (Sometimes a piece might have a range that's not suited to the guitar in a given key.) I think for most people, the difficulty is in reading music with more than a couple of sharps or flats. This can be overcome fairly easily with a bit of practice.

One problem is that a lot of guitar music is in "easy" keys and otherwise mostly in "sharp" keys, so one has to do a bit of extra work to be able to practice this. I've found that a lot of songs in songbooks for voice and piano are also in easy keys, though often in flat keys without many flats. If one really wants to practice keys with lots of sharps or flats, it may be necessary to play from music for piano and adapt it for the guitar. I do this fairly often.

This is for anyone who doesn't already know these rules:

1 flat is F maj. or D min.
1 sharp is G maj. or E min.

For any key signature with more than one flat, the major key is the second-to-last flat, i.e., for two flats, the key is Bb maj. (or G min.) and the two flats are Bb and Eb. For three flats, the flats are Bb, Eb and Ab and the key is Eb maj. (or C min.), etc.

For any key signature with more than one sharp, the maj. key is a half-step above the last sharp. For example, for two sharps, the sharps are F# and C# and the key is D maj. (or B min.). And so on.

So, for the flat keys, the order of major keys according to increasing number of flats is C (0 flats), F, Bb, Eb, Ab, Db, Gb, Cb

For sharp keys, the corresponding order is C, G, D, A, E, B, F#, C#

Thus, the order for the flat keys is the circle of fourths and the order for the sharp keys is the circle of fifths! That means you can use the circle of fourths or fifths to figure out which maj. keys have x number of sharps or flats! Amazing, isn't it?

But wait, there's more! The circle of fourths tells me what chords dominant chords (usually sevenths) resolve to. That is, C7 resolves to F (maj. or min.), F7 resolves to Bb (maj. or min.), etc.

The rule for relative majors and minors is that the relative minor is a sixth above the name of the key for the major (or a minor third below), e.g., A is a sixth above (or a minor third below) C and A min. is the relative minor of C maj. (The relative maj. is therefore a minor third above or a sixth below the name of the minor key.)

Whenever I play something in a key where I don't know these things automatically, I always say them over to myself in my mind first (or out loud), e.g., "Bb, Eb, Ab, Db --- Ab maj. or F min.". Then, if I don't know already, I look to see whether the piece is in the maj. or min. key. Some keys are so common that I don't have to think anymore.

At first, it may be difficult, but one gets faster with practice. Besides, it's not a contest.