Whatever you do, make sure you stick with it so she knows you mean business. You can do it mama, I know you can! Just remember that every time you give in, you've just taught her how long it will take before she gets her way next time. And don't beat yourself up for where you are now. Just decide where you want to be, make a plan and get there. We all make decisions to save our sanity and there's no one right way to parent.
She's old enough that you can start talking with her about how she needs to sleep on her own, and what steps you can take to get there. She may feel empowered by the process enough to "prove" she's a big G. who can handle it. I wouldn't focus on her potential fears (no "what are you afraid of" conversations), but instead state plainly that you and daddy need your own time to talk to each other in the evening, so you can calm down and be ready to sleep as well. And that she needs to learn how to sleep on her own so you can do this.
Talk about what you could do together for her to be calm and ready to sleep on her own -- bath, books, maybe a few extra minutes for stretching, deep breathing, telling a story to a favorite doll or stuffed animal, brushing your hair together -- guide her with suggestions but let her come up with something she feels comfortable doing to start the resting process. My son likes to do breathing and stretching when he's really wound up and it's remarkable how well it works to calm him down.
For the nightmares you could try a couple of things: when you're done reading and she's settled in bed, talk about some things she can dream about that night. I did this when my son first started having bad dreams and it really helped. He'll ask, "What can I dream about tonight?" and we go back and forth with suggestions. It's usually something simple like puppies with wet pink noses, or jumping on the trampolines at the gym. It doesn't really matter, it just gets his thoughts focused on something other than worrying about going to bed.
If she's specifically worried about you disappearing in the night, tell her that you come in and kiss her goodnight every night, before you go to bed yourself. And then really do it (or check on her, if she's a really light sleeper and a kiss would wreck things). When my son is feeling anxious or sick, he'll specifically remind me to come kiss him before I go to bed. Sometimes he gently wakes up and kind of mumbles something, but usually he's passed out cold and has no recollection. But he tells me he remembers it though, which is what matters, I think. It helps him feel safe, knowing that I check in on him.
And lastly, maybe she's ready for a little later bedtime? You said you start her routine at 7:30 but don't mention what time the lights go out. If it's too early, maybe she's just laying awake letting her imagination run wild until she passes out around 9 p.m. If you think this is the case, try moving it up by half an hour and see if it makes a difference. Maybe the new items in her routine could take up this time, or tell her that since she's such a big G. who can now fall asleep on her own that you'll give her an extra half hour of playtime after dinner -- unless you think this would wind her up.
Anyway, good luck! And don't stress about it. The calmer you are about this new process, the calmer she'll be.
PS. my son is about the same age, also an only child and sleeps upstairs while we're downstairs watching T.V. or something. He naps sporadically, with tub time starting around 7:15 and the whole routine wrapping up by 8 p.m., normally. And he's generally asleep within 10 minutes of being in bed with the lights out, and wakes up between 6:30 and 7 a.m.
Edited to Add: Also, you mention that you sit next to her in the room. Part of the Ferber method is moving the chair little by little away from the bed, until you're eventually out of the room. I have friends that did this with small children and they swear by it, because it gradually weans children from relying on your being "right there" but I don't know how it would work with an older child who could get out of bed to try and get your attention.