How to route traffic out a non-standard interface using pfctl?
You could try some kind of re-write script - the tcprewrite tool in linux (it may not exist in OSX) can rewrite packets stored in the a tcpdumpfile. You could then couple this with TCPrelay which, er, replays the re-written tcpdump pcap files.

https://linux.die.net/man/1/tcprewrite

If that seems too much effort (tbh, I could see why) you could try some proxy software that offers some kind of re-write capability. A quick google has shown Charles to be one such, imagine there are others. There is a cost associated with this one, but you can trial it.

https://www.charlesproxy.com/documentation/tools/rewrite/

As I say, I'm sure others are available. No affiliation with Charles, just the first one that I found.