I think I found the culprit, yet I have no idea how to fix it:
Aug 5 19:49:25 pfSense filterlog[41547]: 5,,,1000000104,ovpnc1,match,block,out,4,0x0,,63,0,0,DF,6,tcp,141,10.15.0.2,OUTGOING_IP,13281,443,89,FPA,1717258034:1717258123,761365153,2048,,nop;nop;TS
I see that it also blocks the OPT1 traffic in the system log, as it mentions Default deny rule IPv4 (1000000104).
Does anyone see anything wrong with the instructions I posted in the first post? It doesn't mention any firewall rules on the OPT1 or OpenVPN tab. However, I have come to believe this is no longer correct. Yet when I allow any traffic, it also still doesn't work. It could potentially be asymmetric routing according to the documentation, but I use UDP as a protocol, which it mentions that it shouldn't affect it (https://docs.netgate.com/pfsense/en/latest/troubleshooting/asymmetric-routing.html).
Any help would be greatly appreciated!