Download RuneScape 3 launcher and sandbox it with firejail, with vkBasalt on top.

run.sh

#!/bin/bash
# GPLv2
# Start like this to update RuneScape launcher: UPDATE_RUNESCAPE=1 ./run.sh

#--------------------------------------------
WD=$(realpath "$(dirname "$0")")
runescape=$(realpath "$WD/runescape")
UPDATE_RUNESCAPE=${UPDATE_RUNESCAPE:-0}
if [[ ! $UPDATE_RUNESCAPE -eq 0 ]]; then
    rm -f "$runescape"
fi
if [[ ! -f $runescape ]]; then
    echo "Updating Runescape."
    tmpdir=/tmp/runescape-launcher
    rm -rf "$tmpdir"
    mkdir -p "$tmpdir" || exit 1
    cd "$tmpdir" || exit 1
    curl -Ls -o runescape.PKGBUILD "https://aur.archlinux.org/cgit/aur.git/plain/PKGBUILD?h=runescape-launcher" || exit 1
    pkgver=$(grep ^pkgver runescape.PKGBUILD | cut -d= -f2)
    [[ -z $pkgver ]] && exit 1
    pkgname=$(grep ^pkgname runescape.PKGBUILD | cut -d= -f2)
    [[ -z $pkgname ]] && exit 1
    curl -Ls -o runescape-launcher.deb "https://content.runescape.com/downloads/ubuntu/pool/non-free/r/$pkgname/${pkgname}_${pkgver}_amd64.deb" || exit 1
    7z x runescape-launcher.deb || exit 1
    tar -xf data.tar || exit 1
    cd "$WD" || exit 1
    mv "$tmpdir"/usr/share/games/runescape-launcher/runescape . || exit 1
    rm -rf "$tmpdir"
fi
chmod u+x "$runescape"
ENABLE_VKBASALT=1 MANGOHUD=1 VKBASALT_CONFIG_FILE=~/.config/vkBasalt/vkBasaltRuneScape.conf gamescope --fullscreen -- \
    firejail --nogroups --nodbus --nou2f --private-dev --private-tmp --seccomp \
        --whitelist=~/.runescape --noblacklist=~/.runescape \
        --whitelist=~/Jagex      --noblacklist=~/Jagex \
        --whitelist="$WD"        --noblacklist="$WD" \
        --env=PULSE_LATENCY_MSEC=128 "$runescape"