One of the reasons I started to move towards writing comic book scripts was because I was wanting to write stories where the majority was done from first person, but then having scenes that they weren't in, at the time I felt uncomfortable switching perspectives so thought that scripts would be a good fit as it would allow me to be able to focus on characters individually and being able to switch to all different perspectives very easily.
Switching between 1st/3rd isn't something you guys should worry about though, it's something that is done quite a bit and allows you to keep things fresh and interesting. I always think that when in 1st it's very intriguing when you have an "unreliable narrator" as the reader will question everything they're being told and makes for an excellent read.
As for splitting your books/doing a series I think that's fine so long as your first book has a satisfying ending as a stand alone, but leaves the potential there for more. There's nothing worse than something that is clearly meant just to be a first part and then that next part never gets published, a terrible experience for reader and writer alike as they feel they never got to experience/tell the full story.
I've never done a course specifically on writing (my degree was in English though...and politics, hooray :roll: ) but it's something I would've liked to have done, probably still would. I always keep a small, tight cast of characters in what I write as I try to follow the best rule of writing which is to have the derive the plot from the characters and their motivations and not the other way around. If you've got characters that you don't know what to do with then you're probably best cutting them, if there's attributes you gave them you like then assign them to other characters if you think they'd fit and in doing so would elevate the story. I'd probably recommend against paying for your work to be critiqued, instead show it to someone you trust and whose opinion you value, who you'll think will give you an honest answer. Might be an idea to even go back to an old teacher/lecturer who'd be willing to make the time for you. You could also sign up to a creative writing forum, you'll find a lot of knowledgable pros on some sites who are more than willing to help critique your work. I used to visit one, I think it's
this one, although the site doesn't appear to be working at the moment, at least for me. They also had monthly themed writing competitions, if this is the right site, that I thought was always a good idea.