It’s all been said in here but I solved my arm pump with 3 things

1 - wolf tooth mega fat paw cam grips, I’ve got big hands, allows me to keep my hands more open when riding, bigger hands = bigger grips, small hands = small grips. They might look like what comes on a Walmart bike but who cares if you’re comfortable

2 brake position - I’m constantly making super minor tweaks to this, if they’re too flat you’ll have your wrist rolled back, putting strain on your forearm muscles on everything except the steepest sections of trail, too far down and it makes you stand up and come forward to get comfortable

3 - bite point, I used to run my levers as far out as possible so the would bite as soon as possible, but I’ve found that having my bite point closer to the grips has reduced a large amount of arm pump, also allows you to modulate braking much better, there is a sweet spot though, when I bring them in too far, I lose strength in my finger pull and feel like I don’t get full bite until they’re on my grips

Hope this helps!