The problem with using a venue's piano is that it is often
about as good as a guitar that's been sitting in the cloak
room for 25 years because someone abandoned it there...

I learned piano and 5-string banjo simultaneously, beginning
at age 7. Took up guitar in college, at 20.

My 'formal' training (with music theory) was on piano, and
about 80% of the 'book learning' crossed right over into
fretted instruments, making it easier to do the right things
over there.

Just as nearly every 'educated' (meaning formally educated)
musician is expected to have at least a passing knowledge of
piano, and sight-singing, I believe that it's a great exercise
to also have competency on some fretted instrument.

Also great good fun.

So...if you buy a guitar and take some lessons, you may find
that your piano skill is absolutely not wasted.