Possible to transpose chord symbols ONLY?

Okay, but Marc also wrote:
"The indirect method would be to transpose everything, then just the notes back."

You hadn’t yet described your use case, so I had no way of knowing what it was. For all I know it might well have been that your reason for wanting the chords transposed was for capo purposes. Hence the suggestion.

It's not as simple as just transposing twice; I wish that it was. For example, with a piece in the key of E & chords in C, I had to transpose the piece up to Ab. So then instead of putting my chords into E, it put them into Fb. So then I tried transposing up a major second twice in a row. However, then it started to do a new weird thing in which it would "fix" my key signature after the first line on the page. It would correct the key signature to C for some reason, & then the only fix I would have would be to go line by line thru the whole long piece & add in the correct key signature, which I did not want to do. I had to do a crazy work around in which I had to untick "Transpose key signatures" in order to preserve the key signature in E, transpose the notes & chords up two major seconds in a row to get my chords to E, then cut & paste the notes in the key of E from the original score, then go thru the whole score & delete the extra slurs, because it doubled almost all of my slurs when pasting over. And this is a common issue for me, because I play guitar with a capo & am often trying out different settings, sending a uniform score to my pianist mom, etc. And I don't want to use the capo function, because it lists two chords, the actual & then the capoed key.

I agree with you. It seems like this should be possible from the transposition dialogue. But it's not, so in the meantime it can be done fairly easily.
I must confess that in all the years and groups I've played guitar in, I've always played from lyric sheets with chords. Only using a capo to raise the pitch. I try not to rely on a capo too much because it changes the tone of the instrument. Though I've known plenty of experienced players who use a capo so the don't have to play Bb or Eb.
Just my 2 cents.

If you could just snap your fingers and have it done...

Ah... if only that were possible, then we each could snap our own fingers and have it done. (Sorry, couldn't resist - MuseScore being open source, and the frequent citing of "itch scratching" ;-)

Anyhow...
Regarding your use case No. 2) Say I usually play an accompaniment on guitar in the key of C capo 3.

Do you mean that you play with the capo at the third fret and are "grabbing" a chord shape (i.e., fretboard diagram) as if you were playing in the key of C major? In other words, you "grab" a C chord shape and it sounds as an Eb chord when strummed?
This is commonly seen with those 'open mike folksingers' who "slide the capo up" to suit their individual vocal range. They read the chord symbols written in C major, and play the C major fretboard diagrams so really don't need to view transposed Eb chord symbols. Yes?

I did not say it's too niche to warrant the manpower. I said the reason it hasn't happened yet is that it hasn't been requested often, and that the more it gets requested, the more likely it is to get implemented. And I definitely didn't express any attachment to the current design - I think this dialog is needlessly confusing and awkward to use, and would love to see it overhauled someday.

But if it were up to me, one thing would happen immediately: selecting the chords symbols only and then choosing Tools / Transpose would work. The dialog would work for chord symbols in list selections just as it does for notes, providing an interval option.

As for adding new controls to the dialog, my concern is making an already complex dialog even more complex. So I'd need to see the specific proposed design before commenting on it. Id be much more interested in a design that simplified the dialog rather than one that kep it as is but added yet another control. But sure, if someone came up with a design that were sufficiently clear and didn't impact the normal use cases at all and yet also supported this new functionality, I'd have no problem with it.

To me that is the correct solution "logically", but it's actually more work than just transposing twice.

So as far as I'm concerned, enhancing the transpose dialog to handle chord symbols in list selections is the no-brainer improvement that allows one to save a couple of clicks in this use case without penalizing anyone else by adding yet more controls to the dialog. And I'd recommend submitting this as an issue, after verifying it doesn't already work as you'd expect in current MU4 builds.

A more thorough redesign of the transposition dialog is something I would hope would be considered sometime after 4.0.

But also, back to jeetee's suggestion - if it were possible to add a chord-symbol-only staff (so you didn't need to play games with copy/paste/remove), that would be the better solution still, and it would handle a bunch of other far more common use cases.