There's gonna be so many different answers since everyone's style is different, and for me every comic I do is different, too on how I answer these questions:
1.) How many panels/slides do you like to do per chapter?
It's a webseries, so I kinda don't do chapters anymore. I have...and I kinda still do for my novel (although my chapters are like split into ten parts lol)...but I feel like you don't need to worry about chapter length unless you're doing print. It doesn't need a finite size divisible by 8 for the printer, the books don't need to match in length...you don't have to worry about paper price or any of that.
But, assuming you mean episodes (because 1 page chapters would be a wild experiment), that's different. I'm currently doing a scroll format comic. Scroll formats can be really long, but because I use a lot of colors and gradients, it's a pain in the ass to get it small enough to fit within the MB cap, so I do 15ish panels per episode (some panels are quite long) knowing that each episode is kind of half an episode, but that's fine for my tapas audience. On Webtoon I'll probably upload 2 episodes per update since their MB cap is a little higher and I can upload a little less wide.
But if your scroll format is a lot of flat colors (so no gradients), then this won't matter, you can make pages really, really, really long.
For my page format comic, I did one page per update, and that was fine because I could do 1-2 updates a week, it was black and white. That was done also in western page format, which has more panels than manga since it's an 8.5x11 sized paper (which is also kind of a weird size even for western format, but I was assuming that if I printed it, it would be such a small amount printed that I would just do it on my own, with my own printer or a kinkos somewhere, so I kept it standard page sized), so I tried not to go over 12 panels (which was very rare) but rarely went less than about 5. I averaged about 7 panels per page.
2.) How long does it take to make one full page?
An episode of a scroll format comic takes 1-2 weeks, but that's because it's part time, I don't do it every day, and I don't do it on weekends either. Overall it takes 4-6 hours per episode (sometimes more, depending on if I have to do extra stuff).
I do it all on my own, and right now the longest part is coloring, but I'm getting faster as I go (so it's about 1/3 linework, 2/3 coloring) I have a weird coloring process though, made because I've been doing art a very long time, so I'm not the best person to ask. My method is strange. It's fast, but I had to do a lot of prep work to get it that fast. I think most people spend like 6 hours just coloring, and that's fine. That's actually...pretty understandable, you're doing a lot of panels in an episode. When I started out I was spending like 12 hours on just one illustration. It's...it takes a while to get faster so I wouldn't stress about speed if you are a beginner.
For my b/w comic, because it was very simple and I was doing less panels that were much smaller in size, I could finish a page in 1-3 hours, depending on detail.
3.) How did you come to settle on one idea?
I spend a lot of time scripting. I started my current comic as a NaNoWriMo in 2019, and I played around with it in 2020, so it was a little less than a year of just writing out different ideas until I liked where it was going. I kinda just throw paint at a wall when it comes to scripting. You never know what doesn't work until you try it, so have fun, play around, because once you start making it into a comic--you're kinda cemented in there. It's really hard to change old comic pages, and in my experience, if you have to re-do your beginning, you'll probably end up not finishing the comic and just letting it die. So I think it's worth it to script.
As far as inspiration goes, I think a lot of things inspired me, mostly other works that I enjoy reading. Not that it's a fanfic or anything, but you can see the little easter eggs I've left of what inspired it (especially in character's names).