Congratulations for your PT21, z0diac! I'm very glad that my incomplete runs inspired people.
Please don't overlook PT19, since it's actually not much (if any) harder than the levels you have mentioned currently. The level layout allows many sneaky tactics with teleports, door (near the exit) and a lift (near the exit). Careful and lucky infights is the key here. The only really hard monsters to kill are 1) the cyberdemon and 2) the arch-vile in the yellow key room. Other than that it's not very problematic for a skilled player.
Here are some PT19 observations:
It might be good to try using one of the arch-viles to kill the cyber.
Killing the arch-vile at the yellow key room may take a while with infights, but it's probably the only choice.
It's quite easy to get revenants stuck on the edge of the narrow lift near the exit.
Well, I'm not sure if I can complete any undode tysons, but I'd be glad to contribute with ideas and possibly some incomplete demos. I hope we'll see some new final doom tysons done in the (near) future.
Good luck!
