Trouble relaxing hand on "fast" chords

[newkidintown]

Hello! I hope this is the right board...

I'm working on Liszt's arrangement of Marche au Supplice from Berlioz's Symphonie Fantastique, and I'm having quite a bit of trouble relaxing my hand on the triplet chords at the beginning. I'm fine at low speeds, but as I get closer to the proper tempo my hand gets more and more tense, making the chords sound far too clumsy and heavy.

Having to cross my arms over my body and/or lean to reach the notes (I'm rather short) seems to be contributing, and having to overcome the friction of my nice-but-old piano's keys adds to that. I'm sure there's a better way to reach the low keys without so much contortion, and I'm sure there's something I could do to take some of the work out of pressing the keys, but I'm not sure what. I haven't really had this issue before. Of course, the aforementioned issues could easily have nothing to do with my tension, but I thought I'd mention them in case.

Here's the music, for anyone who's unfamiliar with it: https://petrucci.mus.auth.gr/imglnks/usimg/d/dc/IMSLP68232-PMLP137757-S470B.PDF

[hardy_practice]

Don't let the keys go all the way back up.

[faulty_damper]

When you depress the keys for these chords, are your arms going straight down?  If it is, that's problematic because you have to tense the fingers to brace for impact.  If your fingers are clawed, that further contributes to the tension.  The motion is a slight angle forward.  The wrist will rise during these triplets but you aren't purposely raising them.  It feels incidental.