Issues like that are pretty common and usually not caused by your basic OpenVPN config itself, but by network conditions on your friends’ side.
Most often the problem is ISP routers using strict NAT or blocking certain UDP traffic. Since you’re using udp4, some networks may drop or restrict it. A couple of things to try:
Switch from UDP to TCP (e.g. port 443) — this often works better through restrictive routers/firewalls.
Make sure port forwarding is correctly set on your OPNsense box and that your WAN IP is reachable.
Ask your friends to check if their routers have “VPN passthrough” enabled.
Some ISPs use CG-NAT, which can break incoming connections entirely.
If multiple people have issues, the easiest test is to run the server on TCP 443 — if that suddenly works for everyone, it’s almost certainly router/ISP filtering rather than your config.
