To configure killswitch + (DNS) leak protection + ICMP/ping blocking + protection in case of sys-vpn compromise, you can alternatively execute a three-liner in dom0, no nft/iptables needed:
qvm-firewall sys-vpn reset # (1)
qvm-firewall sys-vpn add accept dsthost=1.2.3.4 # (2)
qvm-firewall sys-vpn del --rule-no 0 # (3)
(1) resets firewall to one single rule accepting everything
(2) whitelists specific VPN gateway IP
(3) removes rule (1), so there is just one whitelisted IP from (2) left
Everything else is blocked safely outside sys-vpn.