I've been looking for a crash fix in the game for 2 months. I played THE FINALS on Void Linux for ~100 hours.
I have 'gamemode' and 'MangoHud' installed with

sudo xbps-install gamemode libgamemode-32bit MangoHud

I tried the next launch options:
1)

gamemoderun mangohud %command%

and ANY other options - crashes in 10-15 minutes
2)

PROTON_LOG_DIR=/tmp PROTON_LOG=1 gamemoderun mangohud %command%

crashes in 25-30 minutes

In order to fix the crashes on Void Linux need to increase vm.max_map_count (https://github.com/polluxau/the-finals-linux-troubleshoot-guide?tab=readme-ov-file#if-your-game-crashes-after-a-hour-or-so) and increase File Descriptor Hard Limit (https://github.com/ValveSoftware/Proton/wiki/File-Descriptors)
Show vm.max_map_count:

sysctl vm.max_map_count

Show File Descriptor Hard Limit:

ulimit -Hn

Increase vm.max_map_count:

sudo sysctl -w vm.max_map_count=2147483642

Increase File Descriptor Hard Limit:

ulimit -Hn 524288

Make settings permanent:

cat << EOF | sudo tee -a /etc/sysctl.conf &> /dev/null

vm.max_map_count=2147483642
EOF

cat << EOF | sudo tee /etc/security/limits.d/90-fd-limits.conf &> /dev/null
*    hard    nofile    524288
EOF

After the crashes were fixed, I ran into a new problem - lag spikes after 25-40 minutes of playing (https://github.com/ValveSoftware/steam-for-linux/issues/11446)
Fix it with the next launch options:

LD_PRELOAD="" %command%

I've been playing for 4 hours now without crashing or any problems