Needed config:
Every traffic should stick using the current 0.0.0.0 default-route via192.168.0.1 except a single IP (192.168.3.55/24) which should next-hop to 10.10.10.1/24 (trough wireguard).
# default route use for all networks
set protocols static route 0.0.0.0/0 next-htop 192.168.0.1
# Default route for PBR
set protocols static table 10 route 0.0.0.0/0 next-hop 10.10.10.1
# Define policies
set policy route FOO rule 10 source address 192.168.3.55
set policy route FOO rule 10 set table 10
# Then attach to interface
set policy route FOO interface eth3 # in 1.4
set interfaces ethernet eth3 policy route FOO # in 1.3