Now I’m thinking about this, I have no idea if whonix is using nftables or iptables. I guess it’s still using iptables so take a look at this guide to allow the port in retroshare qube
see Qubes OS 4.2 nftables / nft firewall guide
The forwarding part you did using iptables in sys-whonix should work I guess, otherwise iptables would have complained.
In your retroshare qube, try sudo nft add rule qubes custom-input tcp dport "30665" accept