I have this issue in PVE, the 1000 character long multiple cast sequences, casts, etc. require you to bind it to like 10 keys and popsicle stick them as fast as you can to pull decent DPS. On key down is good, and there are other alternatives I'm looking into but most look like automation (analog devices... push harder = push faster, doesn't sound legit).

One option is to bind your macros you spam to larger keys (space bar), or even a foot pedal... I knew a guy who'd easiy farm mobs nonstop for 6 hours using a (in game, legit) wow macro that he just used a food pedal for or literally used his fist on the spacebar so as not to tear up his thumbs.

I don't know how your keybinds are setup or how much you mash... but there are a lot of things:
1. Global cooldown - ~1.5 sec delay between instant spells, mashing not necessary
2. Keyboard type and arrangement
3. Position of hands on the keyboard, etc
4. Talk to you doctor about peripheral neuropathy / carpal tunnel / cubital tunnel, etc. if it's that bad and you've tried the rest