https://docs.openwrt.melmac.net/pbr/#rule_create_option README clearly states it works on 21.02 and earlier. Which it does/has been reported to work.
So if you want pbr, install 21.02 with mwan3 and pbr-iptables, change the abovementioned option and it should work. Until mwan3 is rewritten to support nft, there's no way to have (nft-supported) pbr work with it. The pbr in iptables mode or pbr-iptables should theoretically work with mwan3 on 22.03 (especially if you follow advise from @bluewavenet here), but I have not heard of anyone using it this way.
If you have just a few simple rules, you may be better off manually inserting them with iptables commands into the chains mwan3 uses or other chains which mwan3 respects on 22.03.