Okay, so even now when me LO who is 3 months old has slept poorly in the evening I put her to bed super early at 18:30 but usually at 19:00. If she wakes up early from her last nap I never try to keep her up for any longer than her usual A time, it will only make for a horrible evening. This early bedtime shouldn't effect her waking time in the morning, but even if it does, that's something that can probably be altered further ahead. Maybe it is something you should consider, 20:00 seems too late to me to start the bedtime routine. LO's this age should be having bedtime earlier.
What I would try is not keeping her up for longer than an hour after 16:30. Try puttin her down again at 17:30 for another nap and wake her again at 18:15. That would give her another hour until her bedtime, which I would recommend be at the latest 19:15. I don't know what Tracy recommended regarding napping after 17:00 but she has an A time of 1 hour, so she would have to have bedtime at 18:00 if she didn't take another nap...
I think your LO is difficult to put down because her biological clock wants her to be asleep earlier. My LO goes into a kind of a trance when she is ready for bedtime, she is super easy to get to sleep.....
Here is what your evening would look like:
E:16:30
A:
S:17:30
E:18:15
A: bedtime routine
S: 19:15
Give it a try and let me know how you get on!