That's not how routes look when I make them. I should note, personally, I don't use the PBR package, I just make IP routes and rules.
I assisted someone the other day with routing a subnet, so there's no difference, instead you use /32 for the subnet. In fact, this is how I setup single IPs to use various VPNs - then I merely setup connection profiles on the computer and switch as needed - so the OP's use case is the same as mine.
# in /etc/config/network
config route
option interface 'wgc_piaJP'
option target '0.0.0.0/0'
option table '1'
config rule #<--- ensures everything routes to IP when DST
option dest '192.168.1.142/32'
option priority '1'
option lookup 'main'
config rule
option src '192.168.1.142/32'
option dest '0.0.0.0/0'
option priority '2' #order in IP_rules
option lookup '1' #table_assigned
This is how I would make such a config.