2008 orbea ora components

greetings:
apologies in advance if i am creating this post with possible redundancy to exiting other(s).

i have a 2008 orbea ora tri bike. stock components:

  • 10 speed (shimano ultegra FD)
  • changed out the 53-tooth chainring for a 50t. small ring is stock 39t.
  • 10 speed dura-ace RD, 12-25 cassette.

just moved to hilly southern california form flat SE texas. clearly over-geared for my age and local terrain. i’m looking to change out drivetrain components, and would like some guidance/input.

first,i want to put a compact crank up front, 50x34. easy part.
for the rear, though, thinking i want a really low end, greater than the 28t limit on the short-cage RD. can i put a 9-speed long-cage (mountain) RD with a 11/32 9-speed casssette ?

you can see where i’m going with this. any inputs/comments are welcome.
thanks.