Installed 2.3.5. Restored backup. Ran the same battery of speed tests. Instant improvement!
Speed's are still nothing like my non-vpn, but that's as expected. I got 50-80 Mbps on all the test sites that should be VPN. Ironically, speedtest.net which the other day was showing my comcast IP but testing super slow, is now showing my vpn IP, but testing at 276 Mbps. Oh well, at least I'm getting workable speeds through the vpn.
Definitely are some different settings for OpenVPN in 2.4.2 vs 2.3.5. Even though I set them per the guides, apparently something wasn't agreeing with my system.
I think I'll stick with 2.3.5 until I see a real reason to upgrade.