I'm back/am one of the people ITT who had this issue:
I had to COMPLETELY DISABLE Windows Firewall/Defender to allow incoming/outgoing connections.
I had tried configuring it beforehand to allow Minecraft to have all incoming/outgoing connections, I had tried adding port numbers to server IP addresses, none of that worked.
Only disabling the entire Windows Firewall/Defender system could I then connect to other servers.
Seems like overkill/I don't like having my system unsecure for prolonged periods of time, but at least I narrowed in on what the culprit was on my end.