ill try to answer your questions in order based on my opinions ;u;
1. thats up for you to determine tbh. not great advice if youre looking for a solid answer but its something that you decide for yourself over time. you absolutely should consider how much work you put into the designs and how long it takes you
2. advertise on the sites you use most! i do it on deviantart, toyhouse, tumblr (sometimes), and twitter (even rarer). you can use those sites along with commission/art buying-specific sites (i forgot what the were tho lol), and other art sharing/forum sites like paigeeworld, forums (you could put a link in your signatures), and gaia (same signature thing or your profile) or facebook. discord groups are invite-only so you will only get the people in there, and they might not be looking for what you offer so branching out and seeing what sites you like to use would help a lot with advertising. if you upload something you can link your info into the caption and stuff like that
3. my biggest piece of advice is to not start out asking other people what sorts of adopts you should make. someone on here said before that you should make designs that you yourself would want to keep. you can sometimes tell when people make designs they dont care for, because it looks uninspired. when someone is doing something they love and are passionate about you can tell too. use your favorite color palettes! use your favorite shapes! take inspiration from the things most important to you!! once you feel comfortable making designs that you genuinely like you can always ask others for inspiration tho (like "what theme should i try next?" and stuff, but still allow yourself freedom to interpret and design things on your own)
hope any of this helped kdhskdhs