Sounds like you have had an easy time self-publishing Red Wombat. Is my take on it exaggerated?
Kindest Regards,
Winfred
"Easy" is a bit of a loaded term here, so let me start with the basics--I've self-pubbed, I've big-trade-pubbed, I've small-press pubbed, and they're all different and guarantees are few.
My time self-publishing has been a much easier row to hoe than many authors
because I've been trade-published before--I'm a hybrid author. I essentially came to self-pub with a built-in audience that I'd built up over ten years of blogging, art, doing a webcomic, small-press books, etc. (We'll set aside my children's books for the moment--the audience overlap is slim and I don't know how much they're moving the needle on the self-pub stuff.)
At the end of the day, because of my career up to this point, I can pretty much count on a thousand eyeballs minimum on anything I produce.
That is a
lot of eyeballs. (And they lead to more eyeballs!) If I self-publish a book, I can reasonably expect that a thousand people will buy it right out of the gate, because I've poured about fifteen years into somehow fooling those people into thinking I am not a total blithering idiot.*
This is a completely different situation than if this is your first book and you have no audience. First book/no audience, I would ALWAYS suggest going trade if you possibly can. They can get you those eyeballs. Getting them yourself is a LOT of work, and requires either a lot of time or a specialized skillset, and there is absolutely no substitute for a big house with a big marketing team and a budget. Even if you only get the bare bones of that, it's probably more than you can do for yourself.
...this is starting to sound a bit grisly, with all the eyeballs. But anyway!
In the case of the OP here, though, she's GOT an audience--the first two books of the trilogy were trade published, people are clamoring for the last book. That's built-in eyeballs. You usually don't get that out of the gate in self-pub. So OP's third book of a trilogy is a great candidate for self-pub, whereas the first book of a trilogy by an unknown author would be a totally different kettle of fish.
Nobody can say what's right for you, but yeah, a novella is a hard sell. (Tor is currently working with novellas, though, so there's hope.) But if you don't have an audience, I'd generally suggest at least TRYING trade publishing, because you lose nothing by trying it and you could gain a great deal that you can't do for yourself without miserable effort.
*Some of them may be on to me.